"full" implementation of new Py2 API

[davidhewitt]

Closes #3382

This branch has been used to implement pydantic/pydantic-core#1085

It's based on top of #3572, and also includes #3600 as well as updated forms of most of the other PRs introducing Py2 types.

I feel like this strikes a good balance of introducing the new Py2 API without breaking much of the existing gil refs API. The only thing which really needed to change was the FromPyObject trait, to take &Py2<'py, PyAny> instead of &'py PyAny. I don't think I'd want to land this amongst other breaking changes. I do think this might be releasable as an 0.22 release with both APIs still present (maybe gate the GIL refs API with a backcompat feature), with a view to dropping the gil refs API in 0.24 maybe.

Next steps:

• We want to release pydantic-core against a form of this branch in the near future. Any opinions if I should release this as a fork of PyO3, or we release this as an 0.22 alpha?

• I plan to start splitting this out into reviewable PRs and keep this one rebased as we move forwards.

• This also needs further cleanup / documentation etc.

[codspeed-hq]

CodSpeed Performance Report

Merging #3606 will degrade performances by 34.69%

_{Comparing davidhewitt:complete-py2 (c3d9e69) with main (94b7d7e)}

Summary

⚡ 4 improvements
❌ 5 regressions
✅ 70 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	davidhewitt:complete-py2	Change
❌	ordered_dunder_methods	4 µs	6 µs	-33.48%
❌	ordered_richcmp	3.9 µs	6 µs	-34.69%
❌	list_via_downcast	157.2 ns	185 ns	-15.02%
❌	list_via_extract	308.9 ns	364.4 ns	-15.24%
❌	sequence_from_tuple	232.8 ns	260.6 ns	-10.66%
⚡	test_simple_args_kwargs_rs	45.9 µs	38.1 µs	+20.71%
⚡	test_simple_args_rs	39.4 µs	34.9 µs	+12.94%
⚡	test_simple_kwargs_rs	45.6 µs	37.2 µs	+22.55%
⚡	test_simple_rs	35.2 µs	32 µs	+10.02%

[davidhewitt]

This branch has now outlived its usefulness. We've split out basically everything into other PRs, and I see no further need to keep trying to rebase this here for the few remaining odds and ends, so I'll close this. Great work everyone!