Test against Python 3.10 and PyPy3.7

[jwodder]

See actions/setup-python#244 for the latter.

[codecov-commenter]

Codecov Report

Merging #87 (858afb2) into master (655b0f7) will decrease coverage by 0.25%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #87      +/-   ##
==========================================
- Coverage   63.11%   62.85%   -0.26%     
==========================================
  Files           1        1              
  Lines        1163     1163              
  Branches      218      236      +18     
==========================================
- Hits          734      731       -3     
- Misses        364      368       +4     
+ Partials       65       64       -1     

Impacted Files	Coverage Δ
src/datalad_installer.py	62.85% <0.00%> (-0.26%)	⬇️

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[yarikoptic]

Missed this one. It was green. Let's just merge then