add PyDict::new_bound without deprecation

[davidhewitt]

This is split from #3716, it's just the addition of PyDict::new_bound without any deprecation added or accompanying changes.

As this is a tiny diff which repeats exactly the pattern we've already established, I'm just going to merge immediately so other PRs can build on it.

[codspeed-hq]

CodSpeed Performance Report

Merging #3772 will improve performances by 24.75%

_{Comparing davidhewitt:dict-new-bound (d4d08b2) with main (c54d897)}

Summary

⚡ 3 improvements
✅ 77 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	davidhewitt:dict-new-bound	Change
⚡	test_simple_py	34.7 µs	31.4 µs	+10.48%
⚡	extract_int_downcast_fail	266.1 ns	238.3 ns	+11.66%
⚡	mapping_from_dict	420 ns	336.7 ns	+24.75%