Add support for Python 3.13

[ben-c8y]

Including the free-threaded (no-GIL) "t" variant

remove support for the old 3.8; bump the version of several GH actions (some of the old versions no longer work)

This is mostly working, however: a) there's a problem with indicating the pywin32 dependency on windows (not sure what's going on, maybe some changes to pip have broken us) b) on 3.13+ setuptools is behaving differently and not installing the license_files (or at least, not to the same place)... needs debugging, and possibly giving up on that approach for distributing the doc and putting them into a different directory

However packaging issues aside, the tests show everything works (even in GIL-disabled mode)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.43%. Comparing base (dfd40c5) to head (86c2cd9). Report is 43 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #93      +/-   ##
==========================================
+ Coverage   91.40%   91.43%   +0.02%     
==========================================
  Files          65       64       -1     
  Lines        7981     7924      -57     
==========================================
- Hits         7295     7245      -50     
+ Misses        686      679       -7     

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