diff --git a/connexion/__init__.py b/connexion/__init__.py
index fbdc47eac..7b3d7d891 100755
--- a/connexion/__init__.py
+++ b/connexion/__init__.py
@@ -1,15 +1,16 @@
+import sys
+
 import werkzeug.exceptions as exceptions  # NOQA
-from .apps import AbstractApp  # NOQA
+
 from .apis import AbstractAPI  # NOQA
+from .apps import AbstractApp  # NOQA
+from .decorators.produces import NoContent  # NOQA
 from .exceptions import ProblemException  # NOQA
+# add operation for backwards compatability
+from .operations import compat
 from .problem import problem  # NOQA
-from .decorators.produces import NoContent  # NOQA
 from .resolver import Resolution, Resolver, RestyResolver  # NOQA
 
-import sys
-
-# add operation for backwards compatability
-from .operations import compat
 full_name = '{}.operation'.format(__package__)
 sys.modules[full_name] = sys.modules[compat.__name__]
 
diff --git a/connexion/apis/aiohttp_api.py b/connexion/apis/aiohttp_api.py
index e968f253a..aa2c4fb34 100644
--- a/connexion/apis/aiohttp_api.py
+++ b/connexion/apis/aiohttp_api.py
@@ -3,9 +3,8 @@
 import re
 from urllib.parse import parse_qs
 
-import jinja2
-
 import aiohttp_jinja2
+import jinja2
 from aiohttp import web
 from aiohttp.web_exceptions import HTTPNotFound
 from connexion.apis.abstract import AbstractAPI
diff --git a/connexion/apis/flask_api.py b/connexion/apis/flask_api.py
index 83f929f90..dca8b8960 100644
--- a/connexion/apis/flask_api.py
+++ b/connexion/apis/flask_api.py
@@ -3,14 +3,13 @@
 import flask
 import six
 import werkzeug.exceptions
-from werkzeug.local import LocalProxy
-
 from connexion.apis import flask_utils
 from connexion.apis.abstract import AbstractAPI
 from connexion.decorators.produces import NoContent
 from connexion.handlers import AuthErrorHandler
 from connexion.lifecycle import ConnexionRequest, ConnexionResponse
 from connexion.utils import Jsonifier, is_json_mimetype, yamldumper
+from werkzeug.local import LocalProxy
 
 logger = logging.getLogger('connexion.apis.flask_api')
 
diff --git a/connexion/cli.py b/connexion/cli.py
index 74318c534..5394f7efd 100644
--- a/connexion/cli.py
+++ b/connexion/cli.py
@@ -3,9 +3,8 @@
 from os import path
 
 import click
-from clickclick import AliasedGroup, fatal_error
-
 import connexion
+from clickclick import AliasedGroup, fatal_error
 from connexion.mock import MockResolver
 
 logger = logging.getLogger('connexion.cli')
diff --git a/connexion/decorators/security.py b/connexion/decorators/security.py
index 7920fdc54..5ce399ebf 100644
--- a/connexion/decorators/security.py
+++ b/connexion/decorators/security.py
@@ -6,9 +6,8 @@
 import textwrap
 
 import requests
-from six.moves import http_cookies
-
 from connexion.utils import get_function_from_name
+from six.moves import http_cookies
 
 from ..exceptions import (ConnexionException, OAuthProblem,
                           OAuthResponseProblem, OAuthScopeProblem)
diff --git a/connexion/operations/__init__.py b/connexion/operations/__init__.py
index bd1392d07..4c44b9f38 100644
--- a/connexion/operations/__init__.py
+++ b/connexion/operations/__init__.py
@@ -1,7 +1,7 @@
 from .abstract import AbstractOperation  # noqa
 from .openapi import OpenAPIOperation  # noqa
-from .swagger2 import Swagger2Operation  # noqa
 from .secure import SecureOperation  # noqa
+from .swagger2 import Swagger2Operation  # noqa
 
 
 def make_operation(spec, *args, **kwargs):
diff --git a/connexion/operations/abstract.py b/connexion/operations/abstract.py
index 3643df52a..7774407fd 100644
--- a/connexion/operations/abstract.py
+++ b/connexion/operations/abstract.py
@@ -2,7 +2,6 @@
 import logging
 
 import six
-
 from connexion.operations.secure import SecureOperation
 
 from ..decorators.metrics import UWSGIMetricsCollector
diff --git a/tests/aiohttp/test_aiohttp_api_secure.py b/tests/aiohttp/test_aiohttp_api_secure.py
index 86377d189..bb1f5e52c 100644
--- a/tests/aiohttp/test_aiohttp_api_secure.py
+++ b/tests/aiohttp/test_aiohttp_api_secure.py
@@ -2,6 +2,7 @@
 import base64
 
 import ujson
+
 from conftest import TEST_FOLDER
 from connexion import AioHttpApp
 
diff --git a/tests/aiohttp/test_aiohttp_app.py b/tests/aiohttp/test_aiohttp_app.py
index f1c477558..8871e04ca 100644
--- a/tests/aiohttp/test_aiohttp_app.py
+++ b/tests/aiohttp/test_aiohttp_app.py
@@ -2,6 +2,7 @@
 from unittest import mock
 
 import pytest
+
 from conftest import TEST_FOLDER
 from connexion import AioHttpApp
 from connexion.exceptions import ConnexionException
diff --git a/tests/aiohttp/test_aiohttp_simple_api.py b/tests/aiohttp/test_aiohttp_simple_api.py
index 045d3e700..8c51b7b50 100644
--- a/tests/aiohttp/test_aiohttp_simple_api.py
+++ b/tests/aiohttp/test_aiohttp_simple_api.py
@@ -1,10 +1,10 @@
 import asyncio
 import sys
 
+import pytest
 import yaml
 
 import aiohttp.web
-import pytest
 from conftest import TEST_FOLDER
 from connexion import AioHttpApp
 
diff --git a/tests/api/test_bootstrap.py b/tests/api/test_bootstrap.py
index b6eecbc56..da23a3d71 100644
--- a/tests/api/test_bootstrap.py
+++ b/tests/api/test_bootstrap.py
@@ -1,11 +1,11 @@
 import json
 
 import jinja2
+import mock
+import pytest
 import yaml
 from openapi_spec_validator.loaders import ExtendedSafeLoader
 
-import mock
-import pytest
 from conftest import TEST_FOLDER, build_app_from_fixture
 from connexion import App
 from connexion.exceptions import InvalidSpecification
diff --git a/tests/conftest.py b/tests/conftest.py
index c45cb1573..78499ff81 100644
--- a/tests/conftest.py
+++ b/tests/conftest.py
@@ -4,6 +4,7 @@
 import sys
 
 import pytest
+
 from connexion import App
 
 logging.basicConfig(level=logging.DEBUG)
diff --git a/tests/decorators/test_parameter.py b/tests/decorators/test_parameter.py
index e66ab4e4e..8d014b45c 100644
--- a/tests/decorators/test_parameter.py
+++ b/tests/decorators/test_parameter.py
@@ -1,7 +1,8 @@
-from connexion.decorators.parameter import parameter_to_arg
 # we are using "mock" module here for Py 2.7 support
 from mock import MagicMock
 
+from connexion.decorators.parameter import parameter_to_arg
+
 
 def test_injection():
     request = MagicMock(name='request', path_params={'p1': '123'})
diff --git a/tests/decorators/test_security.py b/tests/decorators/test_security.py
index 3679648ed..f5323869d 100644
--- a/tests/decorators/test_security.py
+++ b/tests/decorators/test_security.py
@@ -1,15 +1,15 @@
 import json
 
+import pytest
 import requests
+from mock import MagicMock
 
-import pytest
 from connexion.decorators.security import (get_tokeninfo_func,
                                            get_tokeninfo_remote,
                                            validate_scope, verify_apikey,
                                            verify_basic, verify_oauth)
 from connexion.exceptions import (OAuthProblem, OAuthResponseProblem,
                                   OAuthScopeProblem)
-from mock import MagicMock
 
 
 def test_get_tokeninfo_url(monkeypatch):
diff --git a/tests/decorators/test_uri_parsing.py b/tests/decorators/test_uri_parsing.py
index 27b7f3203..e14420925 100644
--- a/tests/decorators/test_uri_parsing.py
+++ b/tests/decorators/test_uri_parsing.py
@@ -1,6 +1,6 @@
+import pytest
 from werkzeug.datastructures import MultiDict
 
-import pytest
 from connexion.decorators.uri_parsing import (AlwaysMultiURIParser,
                                               FirstValueURIParser,
                                               Swagger2URIParser)
diff --git a/tests/decorators/test_validation.py b/tests/decorators/test_validation.py
index 3576c5dd7..bd51c2d20 100644
--- a/tests/decorators/test_validation.py
+++ b/tests/decorators/test_validation.py
@@ -1,10 +1,10 @@
+import pytest
 from jsonschema import ValidationError
+from mock import MagicMock
 
-import pytest
 from connexion.decorators.validation import ParameterValidator
 from connexion.json_schema import (Draft4RequestValidator,
                                    Draft4ResponseValidator)
-from mock import MagicMock
 
 
 def test_get_valid_parameter():
diff --git a/tests/fakeapi/__init__.py b/tests/fakeapi/__init__.py
index d38cde042..28bc1e50b 100644
--- a/tests/fakeapi/__init__.py
+++ b/tests/fakeapi/__init__.py
@@ -1,4 +1,5 @@
 from .example_method_view import Example_methodView
 
+
 def get():
     return ''
diff --git a/tests/test_api.py b/tests/test_api.py
index 96852f91b..ed1ea90a6 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -3,13 +3,13 @@
 import pathlib
 import tempfile
 
+import pytest
+from mock import MagicMock
 from yaml import YAMLError
 
-import pytest
 from connexion import FlaskApi
 from connexion.exceptions import InvalidSpecification, ResolverError
 from connexion.spec import canonical_base_path
-from mock import MagicMock
 
 TEST_FOLDER = pathlib.Path(__file__).parent
 
diff --git a/tests/test_cli.py b/tests/test_cli.py
index a30ecb3b8..cd7624f53 100644
--- a/tests/test_cli.py
+++ b/tests/test_cli.py
@@ -1,14 +1,14 @@
 import logging
 
+import pytest
 from click.testing import CliRunner
+from mock import MagicMock
+from mock import call as mock_call
 
 import connexion
-import pytest
 from conftest import FIXTURES_FOLDER
 from connexion.cli import main
 from connexion.exceptions import ResolverError
-from mock import MagicMock
-from mock import call as mock_call
 
 
 @pytest.fixture()
diff --git a/tests/test_json_validation.py b/tests/test_json_validation.py
index 18b26149f..0ef01f489 100644
--- a/tests/test_json_validation.py
+++ b/tests/test_json_validation.py
@@ -1,8 +1,8 @@
 import json
 
+import pytest
 from jsonschema.validators import _utils, extend
 
-import pytest
 from conftest import build_app_from_fixture
 from connexion import App
 from connexion.decorators.validation import RequestBodyValidator
diff --git a/tests/test_metrics.py b/tests/test_metrics.py
index 7b753efc2..932243d2b 100644
--- a/tests/test_metrics.py
+++ b/tests/test_metrics.py
@@ -1,10 +1,10 @@
 import json
 
 import flask
+from mock import MagicMock
 
 import connexion
 from connexion.decorators.metrics import UWSGIMetricsCollector
-from mock import MagicMock
 
 
 def test_timer(monkeypatch):
diff --git a/tests/test_operation2.py b/tests/test_operation2.py
index 0e5291c93..983e2ab63 100644
--- a/tests/test_operation2.py
+++ b/tests/test_operation2.py
@@ -5,6 +5,7 @@
 
 import mock
 import pytest
+
 from connexion.apis.flask_api import Jsonifier
 from connexion.decorators.security import (get_tokeninfo_remote,
                                            validate_scope, verify_security)
diff --git a/tests/test_references.py b/tests/test_references.py
index 9e979b6d7..f3bc5388d 100644
--- a/tests/test_references.py
+++ b/tests/test_references.py
@@ -1,5 +1,6 @@
 import mock
 import pytest
+
 from connexion.apis.flask_api import Jsonifier
 from connexion.json_schema import RefResolutionError, resolve_refs
 
diff --git a/tests/test_resolver.py b/tests/test_resolver.py
index 5b17eed8e..fb42b74c3 100644
--- a/tests/test_resolver.py
+++ b/tests/test_resolver.py
@@ -1,5 +1,6 @@
-import connexion.apps
 import pytest
+
+import connexion.apps
 from connexion.exceptions import ResolverError
 from connexion.operations import Swagger2Operation
 from connexion.resolver import Resolver, RestyResolver
diff --git a/tests/test_utils.py b/tests/test_utils.py
index 864c56f8b..a8702ebd7 100644
--- a/tests/test_utils.py
+++ b/tests/test_utils.py
@@ -1,10 +1,11 @@
 import math
 
-import connexion.apps
 import pytest
-from connexion import utils
 from mock import MagicMock
 
+import connexion.apps
+from connexion import utils
+
 
 def test_get_function_from_name():
     function = utils.get_function_from_name('math.ceil')
diff --git a/tests/test_validation.py b/tests/test_validation.py
index 5531a2334..6eded82ec 100644
--- a/tests/test_validation.py
+++ b/tests/test_validation.py
@@ -1,11 +1,11 @@
 import json
 
 import flask
+# we are using "mock" module here for Py 2.7 support
+from mock import MagicMock
 
 from connexion.apis.flask_api import FlaskApi
 from connexion.decorators.validation import ParameterValidator
-# we are using "mock" module here for Py 2.7 support
-from mock import MagicMock
 
 
 def test_parameter_validator(monkeypatch):
diff --git a/tox.ini b/tox.ini
index 4c6affc3d..1697134c1 100644
--- a/tox.ini
+++ b/tox.ini
@@ -43,18 +43,18 @@ commands=python setup.py flake8
 
 [testenv:isort-check]
 basepython=python3
-deps=isort
+deps=isort==4.3.15
 changedir={toxinidir}/connexion
 commands=isort -ns __init__.py -rc -c -df .
 
 [testenv:isort-check-examples]
 basepython=python3
-deps=isort
+deps=isort==4.3.15
 changedir={toxinidir}/examples
 commands=isort -ns __init__.py -o connexion -rc -c -df .
 
 [testenv:isort-check-tests]
 basepython=python3
-deps=isort
+deps=isort==4.3.15
 changedir={toxinidir}/tests
 commands=isort -ns __init__.py -p connexion -rc -c -df .