always use a Python iterator for sets and frozensets

[davidhewitt]

Related to #3762

This removes the branching of Py_LIMITED_API / not(Py_LIMITED_API) code for iterators of set and frozenset, opting to just use the Py_LIMITED_API forms in all cases.

This is slightly slower but recommended for stability due to use of private API _PySet_NextEntry which might also not work properly when iterating set subclasses (if they override iteration).

At the same time I added ExactSizeIterator implementations to both iterators, as we had that previously just for the not(Py_LIMITED_API) case.

Overall this is a code simplification and consistency improvement.

[codspeed-hq]

CodSpeed Performance Report

Merging #3849 will degrade performances by 15.63%

_{Comparing davidhewitt:set-iteration (11bc256) with main (940804f)}

Summary

⚡ 1 improvements
❌ 3 regressions
✅ 75 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	davidhewitt:set-iteration	Change
❌	mapping_from_dict	300 ns	355.6 ns	-15.63%
⚡	f64_from_pyobject	516.7 ns	461.1 ns	+12.05%
❌	sequence_from_list	300 ns	355.6 ns	-15.63%
❌	sequence_from_tuple	232.8 ns	260.6 ns	-10.66%

[davidhewitt]

At least on codspeed, ignoring the usual volatile benchmarks we see reported here, the benchmarks basically show no difference for removing the "fast" path, so I think this increases the justification for doing this.

[LilyFoote]

Seems straightforward to me.

[davidhewitt]

Thanks for the review! 🙏