Always return new references in C Extension

[asvetlov]

Borrowed references to Python objects are dangerous.
The PR constantly uses a new-reference strategy and newer returns a borrowed ref.

[codspeed-hq]

CodSpeed Performance Report

Merging #1084 will degrade performances by 42.42%

_{Comparing strongrefs (7d5e800) with master (5dcfd85)}

Summary

❌ 17 regressions
✅ 36 untouched benchmarks

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

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_cimultidict_extend_istr	129.6 µs	158.6 µs	-18.29%
❌	test_cimultidict_fetch_istr	61.7 µs	85.1 µs	-27.57%
❌	test_cimultidict_fetch_str	86 µs	109.1 µs	-21.14%
❌	test_cimultidict_get_hit	97.4 µs	120.8 µs	-19.39%
❌	test_cimultidict_get_hit_with_default	99.4 µs	122.6 µs	-18.89%
❌	test_cimultidict_get_istr_hit	73.3 µs	97.2 µs	-24.63%
❌	test_cimultidict_get_istr_hit_with_default	75.5 µs	99.3 µs	-23.91%
❌	test_cimultidict_get_istr_miss	77.6 µs	129.5 µs	-40.06%
❌	test_cimultidict_get_istr_with_default_miss	80.1 µs	132.5 µs	-39.55%
❌	test_cimultidict_get_miss	102.6 µs	155.4 µs	-33.95%
❌	test_cimultidict_get_miss_with_default	105.1 µs	156.3 µs	-32.76%
❌	test_multidict_fetch	56.6 µs	79.6 µs	-28.88%
❌	test_multidict_get_hit	61.4 µs	83.1 µs	-26.06%
❌	test_multidict_get_miss	66.9 µs	116.2 µs	-42.42%
❌	test_keys_view_equals	77.6 µs	90.7 µs	-14.46%
❌	test_keys_view_is_disjoint	72.8 µs	97.9 µs	-25.64%
❌	test_keys_view_more_or_equal	74.1 µs	86.8 µs	-14.65%

[asvetlov]

Hmm, very strange. INCREF/DECREF cannot be so slow!

[asvetlov]

Ok, the approach doesn't work for an item finding.
Internal pair_list.h utilities shouldn't incref/decref