Drop experimental Python 3.13 free-threaded

[hugovk]

Free-threaded Python is supported from 3.14 onwards, and has better ecosystem coverage. Many projects are dropping experimental 3.13t support, including cibuildwheel. And it frees a CI slot.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.56%. Comparing base (8a47cb4) to head (a07498e).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #336   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
=======================================
  Files          12       12           
  Lines         913      913           
=======================================
  Hits          909      909           
  Misses          4        4           

Flag	Coverage Δ
macos-latest	97.59% <ø> (ø)
ubuntu-latest	97.59% <ø> (ø)
windows-latest	95.72% <ø> (ø)

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[codspeed-hq]

Merging this PR will not alter performance

✅ 15 untouched benchmarks

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_{Comparing hugovk:rm-3.13t (a07498e) with main (8a47cb4)}