bump inline snapshot, re-enable for freethreaded tests

[davidhewitt]

Change Summary

inline-snapshot 0.20.3 made black formatting thread-safe, let's see if this allows us to re-enable...

Related issue number

N/A

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[bot]]

CodSpeed Performance Report

Merging #1665 will improve performances by 38.12%

_{Comparing dh/free-threaded-snapshot (01c3338) with main (d8be765)}

Summary

⚡ 1 improvements
✅ 156 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_strict_union_error_core	45.1 µs	32.7 µs	+38.12%

[davidhewitt]

Will close this as it looks like inline-snapshot is still not thread-safe. Can resume once upstream is fixed.