Update flake8 requirement from <3.6.0,>=3.5.0 to >=3.5.0,<3.8.0

Updates the requirements on [flake8](https://gitlab.com/pycqa/flake8) to permit the latest version.
- [Release notes](https://gitlab.com/pycqa/flake8/tags)
- [Commits](https://gitlab.com/pycqa/flake8/compare/3.5.0...3.7.7)

Signed-off-by: dependabot[bot] <support@dependabot.com>

setup.py

@@ -28,7 +28,7 @@
     extras_require = {
         "dev": [
             "autopep8 == 1.4.0",
-            "flake8 >= 3.5.0, < 3.6.0",
+            "flake8 >=3.5.0,<3.8.0",
             "flake8-print",
             "pycodestyle >= 2.3.0, < 2.4.0",  # To avoid incompatibility with flake
             ]