implement PyStringMethods

[davidhewitt]

For #3382

There are some interesting bits going on in this PR:

• to_str needs the GILPool to work for abi3 before Python 3.10, so I have added a #[cfg] on the corresponding PyStringMethods API.
• I've introduced to_cow as a new API in PyStringMethods which is an alternative for users on abi3 before 3.10 (cc @alex)
• I've added Py::to_str, Py::to_cow and Py::to_string_lossy as reads which can outlast the GIL lifetime, like we did for bytes (cf GIL-free references to Python data #2275)

[alex]

Makes sense to me.

[codspeed-hq]

CodSpeed Performance Report

Merging #3677 will degrade performances by 33.39%

_{Comparing davidhewitt:string2 (f4f3169) with main (8bd2972)}

Summary

❌ 1 regressions
✅ 77 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	davidhewitt:string2	Change
❌	extract_str_downcast_success	422.2 ns	633.9 ns	-33.39%

[davidhewitt]

We've had a little bit of flakiness with PyPy 3.8 previously so will retry in the hope it's not caused by this PR.

[davidhewitt]

Hmm I'll try to investigate this later 🤔

[davidhewitt]

I noticed that PyPy 3.8 succeeded in the second run (but failed in the first), so I think this is just the usual PyPy flakiness :(