remove internal uses of _Py_NewRef

[davidhewitt]

This is a part of #3762, specifically removing _Py_NewRef as something which PyO3 uses internally. It was straightforward to do this using safe APIs.

[codspeed-hq]

CodSpeed Performance Report

Merging #3765 will degrade performances by 10.44%

_{Comparing davidhewitt:remove-py-newref (87e0610) with main (f449fc0)}

Summary

❌ 1 regressions
✅ 77 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	main	davidhewitt:remove-py-newref	Change
❌	extract_int_downcast_fail	238.3 ns	266.1 ns	-10.44%