Add Python 3.13t (free-threaded) builds

[effigies]

The Cython alpha release hit PyPI a couple days ago. You may prefer to wait until there's a stable build, but in case you're interested, the tests pass locally in the x86_64 linux build.

If you're up for this, would you also want a full test suite in main.yaml/pull_request.yaml? We could install a free-threaded build with conda, deadsnakes or uv.

[pauldmccarthy]

Hi @effigies, thanks - I'm inclined to not bother adding a full test suite at this point in time, but could do in the future if there appears to be a need for it.

The IndexedGzipFile class protects against concurrent access, so there aren't any performance gains to be made by using the free-threading build, but there may be possible future enhancements (although it may make more sense to look at rapidgzip).

I'll merge this in, and will get a new release out this week.