Add Python 3.10 to tox config

[frenzymadness]

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[codecov]

Codecov Report

Merging #465 (bde1255) into master (1bf5277) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #465   +/-   ##
=======================================
  Coverage   92.56%   92.56%           
=======================================
  Files           4        4           
  Lines         713      713           
  Branches      156      156           
=======================================
  Hits          660      660           
  Misses         32       32           
  Partials       21       21           

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[ogrisel]

Thanks. We could also update the CI config to make the 3.10 build not "dev" anymore and add 3.11 as dev instead (along with the fixes for #466) in a subsequent PR.