ci: fix pypy & use Python 3.10.0

[henryiii]

See https://github.com/actions/setup-python. PyPy 3.6 is no longer supported, so using 3.7.

[codecov]

Codecov Report

Merging #423 (83d8147) into main (7ab18e4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #423   +/-   ##
=======================================
  Coverage   80.69%   80.69%           
=======================================
  Files          12       12           
  Lines         974      974           
=======================================
  Hits          786      786           
  Misses        188      188           

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[agronholm]

Thanks!