deprecate Py::as_ref

[LilyFoote]

I think rewriting the docs to use the new API can happen in a different PR. The other outstanding bits I think will also be tidied up in other work.

[codspeed-hq]

CodSpeed Performance Report

Merging #3864 will improve performances by 13.43%

_{Comparing LilyFoote:deprecate-py-as-ref (d212c64) with main (a15e4b1)}

Summary

⚡ 2 improvements
✅ 65 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	LilyFoote:deprecate-py-as-ref	Change
⚡	test_simple_args_kwargs_rs	35.3 µs	31.8 µs	+11.22%
⚡	test_simple_kwargs_rs	35 µs	30.9 µs	+13.43%

[davidhewitt]

Thanks!

Agreed re docs can be follow-up; for the remaining pieces I've left some suggestions...

[davidhewitt]

This looks great to me, thanks! 🚀

[davidhewitt]

👍 yes I'm looking forward to using format argument capture when we bump MSRV 😅