ci: use matrix for Python versions

[P403n1x87]

With this change, we can use the install-python action for installing Python on all supported platforms.

[codecov]

Codecov Report

Merging #120 (3978aee) into devel (d86921b) will decrease coverage by 0.68%.
The diff coverage is n/a.

❗ Current head 3978aee differs from pull request most recent head 9e16e3c. Consider uploading reports for the commit 9e16e3c to get more accurate results

@@            Coverage Diff             @@
##            devel     #120      +/-   ##
==========================================
- Coverage   69.83%   69.15%   -0.69%     
==========================================
  Files          22       22              
  Lines        2347     2347              
  Branches      655      655              
==========================================
- Hits         1639     1623      -16     
- Misses        411      421      +10     
- Partials      297      303       +6     

Impacted Files	Coverage Δ
src/py_string.h	67.94% <0.00%> (-15.39%)	⬇️
src/stack.h	81.29% <0.00%> (-2.88%)	⬇️
src/py_thread.c	71.86% <0.00%> (-1.74%)	⬇️
src/py_proc.c	61.16% <0.00%> (+1.21%)	⬆️
src/stats.c	95.55% <0.00%> (+6.66%)	⬆️

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[P403n1x87]

We cannot use the action on Linux because of issue actions/setup-python#442. This will be fixed later.