Add Python 3.10 support

[ymyzk]

Description

• Add Python 3.10 support: https://www.python.org/dev/peps/pep-0619/
  • Update CI config
  • Update package metadata

Expected Behavior

tox-gh-actions runs on Python 3.10 as well without issues.

[codecov-commenter]

Codecov Report

Merging #87 (6013617) into master (687139d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   86.31%   86.31%           
=======================================
  Files           2        2           
  Lines          95       95           
  Branches       25       25           
=======================================
  Hits           82       82           
  Misses          6        6           
  Partials        7        7           

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