add test run for 3.13t

[davidhewitt]

Change Summary

Initial attempt at testing free-threaded Python. Looks like there's quite a few kinks to work out before we can call this stable. Let's run this here in CI, assess how things look, and decide whether to merge as-is (allowing the failures) or to fix before merging.

cc @Viicos

Related issue number

Xref #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 #1626 will not alter performance

_{Comparing dh/3.13t (d1b277d) with main (0862ea5)}

Summary

✅ 157 untouched benchmarks

[davidhewitt]

@Viicos I will merge this now so that we can have the freelist patch applied in the next pydantic-core release so that downstream pydantic can at least start running tests and see what happens.

[davidhewitt]

The test run will fail on every build but does not block merge.