Add support for Python 3.10

[hugovk]

Python 3.10 release candidate is out, and 3.10-dev has been tested for some time, so let's declare support.

Changes proposed in this pull request:

• Add Trove classifier
• Update tox.ini

[codecov-commenter]

Codecov Report

Merging #223 (d7fca7b) into master (d4c3d86) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #223   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files           9        9           
  Lines         632      632           
=======================================
  Hits          630      630           
  Misses          2        2           

Flag	Coverage Δ
GHA_Ubuntu	98.41% <ø> (ø)
GHA_Windows	98.10% <ø> (ø)
GHA_macOS	98.41% <ø> (ø)

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