support &Bound<PyType> for classmethod

[davidhewitt]

Build on stream this morning; there's a little bit of tidy up to do later.

This solves #3705 (comment) by introducing some helper functions in impl_/pymethods.rs

[codspeed-hq]

CodSpeed Performance Report

Merging #3744 will degrade performances by 10.44%

_{Comparing davidhewitt:classmethod (92c8203) with main (43504cd)}

Summary

⚡ 2 improvements
❌ 1 regressions
✅ 75 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	davidhewitt:classmethod	Change
❌	extract_int_downcast_fail	238.3 ns	266.1 ns	-10.44%
⚡	not_a_list_via_downcast	153.9 ns	126.1 ns	+22.03%
⚡	list_via_extract	303.3 ns	275.6 ns	+10.08%

[LilyFoote]

@davidhewitt Instead of doing it from scratch on #3894, I'm going to rebase this and tidy up.

[davidhewitt]

👍 Note that a large part of this was split into #3831

[davidhewitt]

Rest is now in #3897. Thanks @LilyFoote!