Add Python 3.10 support

[gdubicki]

...and also:

• Switch from Python 3.8 to 3.9 as the only Python version for building and security testing,
• Switch from old Coveralls to new Codecov.io,
• Remove tox as we use GitHub Actions matrix to run tests on multiple Python versions,
• Switch from unmaintained pytest-pep8 to a working pytest-flake8 (required for Pytest upgrade >= 6),
• Fix some PEP8 style issues,
• Make mypy static types tests actually enforced,

[coveralls]

Coverage remained the same at 84.529% when pulling e5ce4a4 on gdubicki:add_python_3.10 into 1c6a26e on voxpupuli:master.

[codecov]

Codecov Report

❗ No coverage uploaded for pull request base (master@1c6a26e). Click here to learn what that means.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master     #213   +/-   ##
=========================================
  Coverage          ?   84.95%           
=========================================
  Files             ?       14           
  Lines             ?      917           
  Branches          ?        0           
=========================================
  Hits              ?      779           
  Misses            ?      138           
  Partials          ?        0           

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