Support freethreaded Python 3.14

[seddonym]

Added testing for freethreaded Python 3.14, together with wheel-building on release.

3.13t is not supported by PyO3, so we don't support it either.

[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 23 skipped benchmarks¹

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_{Comparing freethreading-support (f1a4acb) with main (8fdab7d)}

Footnotes

1. 23 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩