support free-threaded Python 3.13t

[davidhewitt]

Change Summary

Adds the PyO3 flag #[pymodule(gil_used = false)] to declare full support for free-threading.

Also adds wheel builds for 3.13t.

(I couldn't help myself doing a drive-by cleanup to the #[pymodule] to use more modern PyO3 syntax at the same time.)

cc @ngoldbaum

Related issue number

Closes #1555

Checklist

• Unit tests for the changes exist
• Documentation reflects the changes where applicable
• Pydantic tests pass with this pydantic-core (except for expected changes)
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #1628 will not alter performance

_{Comparing dh/support-free-threaded (a3e3911) with main (db63fec)}

Summary

✅ 157 untouched benchmarks

[davidhewitt]

Removed a needless use of unsafe code while I'm at it (probably predates the addition of an API in PyO3 to do the same).