Add support for Python 3.10

[hugovk]

Thanks for submitting a PR!

Please make sure to check for the following items:

• Add unit tests and integration tests where applicable.
  If you've added an error code or changed an error code behavior,
  you should probably add or change a test case file under tests/test_cases/ and add
  it to the list under tests/test_definitions.py.
  If you've added or changed a command line option,
  you should probably add or change a test in tests/test_integration.py.
• Add a line to the release notes (docs/release_notes.rst) under "Current Development Version".
  Make sure to include the PR number after you open and get one.

Please don't get discouraged as it may take a while to get a review.

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• Bump pytest to 6.2, the min version supporting 3.10.
  • Maybe use modern
• Ditch pytest-pep8 from tests, it's unmaintained (last release 2014) and doesn't support pytest 6.2.
• Drop the dot in py.test https://twitter.com/pytestdotorg/status/753767547866972160

[samj1912]

Thanks for the PR! Will cut a release next week :)