implement PyErr::write_unraisable_bound

[davidhewitt]

In the spirit of the suggestion to go for the deprecate-and-add-bound-variant approach, here's a PR which implements this change for PyErr::write_unraisable, as I bumped into this when implementing #3697

[adamreichold]

LGTM. One nit in python_format.

[codspeed-hq]

CodSpeed Performance Report

Merging #3698 will improve performances by 22.03%

_{Comparing davidhewitt:unraisable-bound (1004ffa) with main (214ed29)}

Summary

⚡ 1 improvements
✅ 77 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	davidhewitt:unraisable-bound	Change
⚡	not_a_list_via_downcast	153.9 ns	126.1 ns	+22.03%

[davidhewitt]

Rebased to fix the compile errors.

FYI I added a commit to export Bound and Borrowed as pyo3::Bound and pyo3::Borrowed so that the doctests and macros can work outside of the crate. I don't think that's controversial so I'll proceed with the merge, we can adjust this if there are other opinions about what their public path should be.