implement datetime traits for Bound

[davidhewitt]

Split from #3606

This works a bit differently from the other PyXMethods traits, because we already have traits for the datetime types, so we can just implement them for Bound types.

The only slight snag is that PyTzInfoAccess returns a gil-ref, so for now I've added PyTzInfoAccess2 (and we can clean that up when we make the new API public) I've had to modify that to return a Bound tzinfo instead.

[codspeed-hq]

CodSpeed Performance Report

Merging #3679 will improve performances by 22.03%

_{Comparing davidhewitt:datetime2 (6832bf8) with main (65f25d4)}

🎉 Hooray! pytest-codspeed just leveled up to 2.2.0!

A heads-up, this is a breaking change and it might affect your current performance baseline a bit. But here's the exciting part - it's packed with new, cool features and promises improved result stability 🥳!
Curious about what's new? Visit our releases page to delve into all the awesome details about this new version.

Summary

⚡ 1 improvements
✅ 77 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	davidhewitt:datetime2	Change
⚡	list_via_extract	153.9 ns	126.1 ns	+22.03%

[adamreichold]

LGTM. The CI says the pytests still need to be adjusted though.

[davidhewitt]

Thanks for fixing those up! I ended up getting a bit deep into rebasing #3606 for the new Bound / Borrowed names; that branch is looking a lot tidier now!