Add support for Python 3.10

[hugovk]

Changes proposed in this pull request:

• Add Python 3.10 classifier
• Wheel builder: Update manylinux1 to manylinux2014 to build Python 3.10 wheels
• Test Python 3.10-dev on all OS
• Test PyPy3 also on Windows

TODO:

• Adjust required branch status checks before merge.

[codecov-commenter]

Codecov Report

Merging #472 (cd897fd) into main (f7dc48a) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #472   +/-   ##
=======================================
  Coverage   88.65%   88.65%           
=======================================
  Files           6        6           
  Lines        1631     1631           
=======================================
  Hits         1446     1446           
  Misses        185      185           

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