Python 3.15b1 ABI support

[ngoldbaum]

towards fixing #6012.

Adds support for the PEP 820 PySlot API, updates the PEP 793 implementation, and adds the critical section API to the limited API. Also fixes a small change to PyTypeObject that crept in since alpha8.

Claude helped a bit for code review and to figure out the PySlot_FUNC and the __pyo3_pymodexport macros.

Ping @bschoenmaeckers because you asked in the issue what this would look like.

[ngoldbaum]

It's also possible to do this with a sealed trait but that felt magical and complicated for a tiny bit of type safety that felt questionably useful to me, in practice. Please let me know if you'd prefer I not launder an unsafe transmute inside a macro like this.

[codspeed-hq]

Merging this PR will improve performance by 12.89%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 104 untouched benchmarks
⏩ 1 skipped benchmark¹

Performance Changes

	Benchmark	BASE	HEAD	Efficiency
⚡	bench_pyclass_create	4.6 µs	4.1 µs	+12.89%

---

_{Comparing ngoldbaum:pep-820 (117205b) with main (f57bda7)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[ngoldbaum]

I added 3.15 and 3.15t CI to the PR CI builds and the new CI is passing. I just rebased and squashed to get a better coverage report. (EDIT: I guess it makes sense the coverage CI is failing, given the new untested FFI definitions)

That said, this is definitely ready for review. Please let me know if you'd prefer I do this in smaller chunks.