Fixes #13840.

There are some choices to makes, whether to test: 3.13t, 3.14t, and/or 3.15t to start with.

I left out 3.13t because free threading is experimental in 3.13, and projects like cibuildwheel and manylinux are dropping it soon, so many projects will stop producing 3.13t wheels:

• https://github.com/pypa/cibuildwheel/releases/tag/v3.4.1
• Drop CPython 3.13 free-threading cibuildwheel#2683
• Drop CPython 3.8 & 3.13t on 2026-05-05 manylinux#1882

I left out 3.15t because cryptography doesn't yet have wheels for it, so fail to build from source. They'll have wheels once the 3.15 ABI is stable.

• https://github.com/hugovk/pip/actions/runs/24603945780/job/71947042467
• Cannot build on 3.15t pyca/cryptography#13974

And whether to test on Linux, Mac, and/or Windows.

I left out Windows, because the "Windows 1" job takes over half an hour, when everything else is under 10 minutes.

Also whether to add classifiers, like packaging has: pypa/packaging#1098 (changes)

Up to you! I left out classifiers, happy to add whichever you like.