Bump PyO3 to 0.23, drop Python 3.8 support

[gi0baro]

This also enables free-threaded Python support, but with placeholders in CI due to lack of support in the relevant actions.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.05%. Comparing base (a81a7c6) to head (f865404).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #307      +/-   ##
==========================================
- Coverage   99.51%   99.05%   -0.47%     
==========================================
  Files           6        6              
  Lines         416      423       +7     
  Branches       77       76       -1     
==========================================
+ Hits          414      419       +5     
- Misses          1        2       +1     
- Partials        1        2       +1     

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[samuelcolvin]

Thanks so much.