diff --git a/.circleci/config.yml b/.circleci/config.yml
index 2df0719c..bb0aa10e 100644
--- a/.circleci/config.yml
+++ b/.circleci/config.yml
@@ -16,14 +16,6 @@ workflows:
               ignore: /.*/
             tags:
               only: /^v\d+\.\d+\.\d+.*/
-      - unit-3.5:
-          filters:
-            tags:
-              only: /.*/
-      - unit-cpp-3.5:
-          filters:
-            tags:
-              only: /.*/
       - unit-3.6:
           filters:
             tags:
@@ -48,20 +40,28 @@ workflows:
           filters:
             tags:
               only: /.*/
+      - unit-3.9:
+          filters:
+            tags:
+              only: /.*/
+      # - unit-cpp-3.9:
+      #     filters:
+      #       tags:
+      #         only: /.*/
       - docs:
           filters:
             tags:
               only: /.*/
       - publish:
           requires:
-            - unit-3.5
-            - unit-cpp-3.5
             - unit-3.6
             - unit-cpp-3.6
             - unit-3.7
             - unit-cpp-3.7
             - unit-3.8
             - unit-cpp-3.8
+            - unit-3.9
+            # - unit-cpp-3.9
             - docs
           filters:
             branches:
@@ -69,9 +69,9 @@ workflows:
             tags:
               only: /^\d+\.\d+\.\d+((a|b|rc)\d+)?$/
 jobs:
-  unit-3.5:
+  unit-3.6:
     docker:
-      - image: python:3.5
+      - image: python:3.6
     steps:
       - checkout
       - run:
@@ -81,14 +81,14 @@ jobs:
             pip install codecov
       - run:
           name: Run unit tests.
-          command: nox -s unit-3.5
+          command: nox -s unit-3.6
       - run:
           name: Submit coverage data to codecov.
           command: codecov
           when: always
-  unit-3.6:
+  unit-3.7:
     docker:
-      - image: python:3.6
+      - image: python:3.7
     steps:
       - checkout
       - run:
@@ -98,14 +98,14 @@ jobs:
             pip install codecov
       - run:
           name: Run unit tests.
-          command: nox -s unit-3.6
+          command: nox -s unit-3.7
       - run:
           name: Submit coverage data to codecov.
           command: codecov
           when: always
-  unit-3.7:
+  unit-3.8:
     docker:
-      - image: python:3.7
+      - image: python:3.8
     steps:
       - checkout
       - run:
@@ -115,14 +115,14 @@ jobs:
             pip install codecov
       - run:
           name: Run unit tests.
-          command: nox -s unit-3.7
+          command: nox -s unit-3.8
       - run:
           name: Submit coverage data to codecov.
           command: codecov
           when: always
-  unit-3.8:
+  unit-3.9:
     docker:
-      - image: python:3.8
+      - image: python:3.9
     steps:
       - checkout
       - run:
@@ -132,14 +132,14 @@ jobs:
             pip install codecov
       - run:
           name: Run unit tests.
-          command: nox -s unit-3.8
+          command: nox -s unit-3.9
       - run:
           name: Submit coverage data to codecov.
           command: codecov
           when: always
-  unit-cpp-3.5:
+  unit-3.10:
     docker:
-      - image: python:3.5
+      - image: python:3.10-rc
     steps:
       - checkout
       - run:
@@ -149,7 +149,7 @@ jobs:
             pip install codecov
       - run:
           name: Run unit tests.
-          command: nox -s unitcpp-3.5
+          command: nox -s unit-3.10
       - run:
           name: Submit coverage data to codecov.
           command: codecov
@@ -205,6 +205,23 @@ jobs:
           name: Submit coverage data to codecov.
           command: codecov
           when: always
+  # unit-cpp-3.9:
+  #   docker:
+  #     - image: python:3.9
+  #   steps:
+  #     - checkout
+  #     - run:
+  #         name: Install nox and codecov.
+  #         command: |
+  #           pip install nox
+  #           pip install codecov
+  #     - run:
+  #         name: Run unit tests.
+  #         command: nox -s unitcpp-3.9
+  #     - run:
+  #         name: Submit coverage data to codecov.
+  #         command: codecov
+  #         when: always
   docs:
     docker:
       - image: python:3.6
diff --git a/.github/sync-repo-settings.yaml b/.github/sync-repo-settings.yaml
index 9d1600c8..fec40052 100644
--- a/.github/sync-repo-settings.yaml
+++ b/.github/sync-repo-settings.yaml
@@ -7,11 +7,10 @@ branchProtectionRules:
   requiredStatusCheckContexts:
     - 'ci/circleci: docs'
     - 'ci/circleci: style-check'
-    - 'ci/circleci: unit-3.5'
     - 'ci/circleci: unit-3.6'
     - 'ci/circleci: unit-3.7'
     - 'ci/circleci: unit-3.8'
-    - 'ci/circleci: unit-cpp-3.5'
+    - 'ci/circleci: unit-3.9'
     - 'ci/circleci: unit-cpp-3.6'
     - 'ci/circleci: unit-cpp-3.7'
     - 'ci/circleci: unit-cpp-3.8'
diff --git a/noxfile.py b/noxfile.py
index 3a2fff28..0ed6caa7 100644
--- a/noxfile.py
+++ b/noxfile.py
@@ -14,17 +14,25 @@
 
 from __future__ import absolute_import
 import os
+import pathlib
 
 import nox
 
 
-@nox.session(python=["3.5", "3.6", "3.7", "3.8"])
+CURRENT_DIRECTORY = pathlib.Path(__file__).parent.absolute()
+
+
+@nox.session(python=["3.6", "3.7", "3.8", "3.9"])
 def unit(session, proto="python"):
     """Run the unit test suite."""
 
+    constraints_path = str(
+        CURRENT_DIRECTORY / "testing" / f"constraints-{session.python}.txt"
+    )
+
     session.env["PROTOCOL_BUFFERS_PYTHON_IMPLEMENTATION"] = proto
     session.install("coverage", "pytest", "pytest-cov", "pytz")
-    session.install("-e", ".[testing]")
+    session.install("-e", ".[testing]", "-c", constraints_path)
 
     session.run(
         "py.test",
@@ -43,7 +51,10 @@ def unit(session, proto="python"):
     )
 
 
-@nox.session(python=["3.5", "3.6", "3.7", "3.8"])
+# Check if protobuf has released wheels for new python versions
+# https://pypi.org/project/protobuf/#files
+# This list will generally be shorter than 'unit'
+@nox.session(python=["3.6", "3.7", "3.8"])
 def unitcpp(session):
     return unit(session, proto="cpp")
 
diff --git a/setup.py b/setup.py
index 1d17e85d..83bd76f9 100644
--- a/setup.py
+++ b/setup.py
@@ -38,6 +38,7 @@
     include_package_data=True,
     install_requires=("protobuf >= 3.12.0",),
     extras_require={"testing": ["google-api-core[grpc] >= 1.22.2",],},
+    python_requires=">=3.6",
     classifiers=[
         "Development Status :: 5 - Production/Stable",
         "Environment :: Console",
@@ -45,10 +46,10 @@
         "License :: OSI Approved :: Apache Software License",
         "Operating System :: POSIX",
         "Programming Language :: Python :: 3",
-        "Programming Language :: Python :: 3.5",
         "Programming Language :: Python :: 3.6",
         "Programming Language :: Python :: 3.7",
         "Programming Language :: Python :: 3.8",
+        "Programming Language :: Python :: 3.9",
         "Topic :: Software Development :: Code Generators",
         "Topic :: Software Development :: Libraries :: Python Modules",
     ],
diff --git a/testing/constraints-3.10.txt b/testing/constraints-3.10.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/testing/constraints-3.11.txt b/testing/constraints-3.11.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/testing/constraints-3.6.txt b/testing/constraints-3.6.txt
new file mode 100644
index 00000000..f7365f65
--- /dev/null
+++ b/testing/constraints-3.6.txt
@@ -0,0 +1,9 @@
+# This constraints file is used to check that lower bounds
+# are correct in setup.py
+# List *all* library dependencies and extras in this file.
+# Pin the version to the lower bound.
+#
+# e.g., if setup.py has "foo >= 1.14.0, < 2.0.0dev",
+# Then this file should have foo==1.14.0
+protobuf==3.12.0
+google-api-core==1.22.2
\ No newline at end of file
diff --git a/testing/constraints-3.7.txt b/testing/constraints-3.7.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/testing/constraints-3.8.txt b/testing/constraints-3.8.txt
new file mode 100644
index 00000000..e69de29b
diff --git a/testing/constraints-3.9.txt b/testing/constraints-3.9.txt
new file mode 100644
index 00000000..e69de29b