squash! Add support for Python 3.12

[fghaas]

Also, update the GitHub Actions workflow matrix so that we test all supported pip versions with Python 3.8–3.11, but only pip 23 on Python 3.12.

[fghaas]

Feel free to pull this one into your topic branch and then squash it, before you proceed with cleura#282. That is, assuming the tests pass. :)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.43%. Comparing base (70c216d) to head (d2163db).

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Additional details and impacted files

@@             Coverage Diff              @@
##           python-3.12       #8   +/-   ##
============================================
  Coverage        95.43%   95.43%           
============================================
  Files               24       24           
  Lines             1972     1972           
============================================
  Hits              1882     1882           
  Misses              90       90           

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[mrtmm]

Ack, thanks!