feat: add 3.9 support and drop 3.5

[busunkim96]

• Drop 3.5 which is now EOL. https://www.python.org/downloads/release/python-3510/
• Add 3.9 tests and make them required.
• Add a constraints file and use it to test lower bounds in the setup.py in one unit test session (3.6)

[codecov]

Codecov Report

Merging #167 (80802db) into master (5c14cba) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##            master      #167   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           20        20           
  Lines          862       862           
  Branches       149       149           
=========================================
  Hits           862       862           

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[busunkim96]

3.9 failures looks legitimate. Does some extra package need to be installed? @software-dov

https://app.circleci.com/pipelines/github/googleapis/proto-plus-python/409/workflows/059ae846-628f-498d-a500-68ca5ba92dca/jobs/3647/parallel-runs/0/steps/0-103?invite=true

nox > Running session unitcpp-3.9
nox > Creating virtual environment (virtualenv) using python3.9 in .nox/unitcpp-3-9
nox > pip install coverage pytest pytest-cov pytz
nox > pip install -e .[testing] -c /root/project/testing/constraints-3.9.txt
nox > py.test -W=error --quiet --cov=proto --cov-config=.coveragerc --cov-report=term --cov-report=html tests/
ImportError while loading conftest '/root/project/tests/conftest.py'.
tests/conftest.py:18: in <module>
    from google.protobuf import descriptor_pool
.nox/unitcpp-3-9/lib/python3.9/site-packages/google/protobuf/descriptor_pool.py:63: in <module>
    from google.protobuf import descriptor
.nox/unitcpp-3-9/lib/python3.9/site-packages/google/protobuf/descriptor.py:48: in <module>
    from google.protobuf.pyext import _message
E   ImportError: cannot import name '_message' from 'google.protobuf.pyext' (/root/project/.nox/unitcpp-3-9/lib/python3.9/site-packages/google/protobuf/pyext/__init__.py)
nox > Command py.test -W=error --quiet --cov=proto --cov-config=.coveragerc --cov-report=term --cov-report=html tests/ failed with exit code 4
nox > Session unitcpp-3.9 failed.

[software-dov]

I ran into a similar problem when I initially tried running with 3.9; my best guess is that it's indicative of missing 3.9-specific protobuf C++ libs. We should double check with the python protobuf folks to see if that is an artifact that needs to be released.

[busunkim96]

Looks like wheels are not yet published for 3.9. protocolbuffers/protobuf#7978

https://pypi.org/project/protobuf/3.14.0/#files has up through 3.8.

[busunkim96]

I commented out the 3.9 C++ session for now.