rename CI workflows

[mlin]

More-clearly separate Python and R workflows.

Just renames the .yml files with py- and r- prefixes. I tried making .github/workflows/{python,r} subdirectories, but unfortunately this didn't work, GitHub seemed to ignore them.

[codecov]

Codecov Report

Merging #232 (b7f8258) into main (9097162) will not change coverage.
The diff coverage is n/a.

❗ Current head b7f8258 differs from pull request most recent head 875439c. Consider uploading reports for the commit 875439c to get more accurate results

@@           Coverage Diff           @@
##             main     #232   +/-   ##
=======================================
  Coverage   83.75%   83.75%           
=======================================
  Files          29       29           
  Lines        1619     1619           
=======================================
  Hits         1356     1356           
  Misses        263      263           

Flag	Coverage Δ
unittests	83.75% <ø> (ø)

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