Get rid of borrowed references in getters

[asvetlov]

The PR doesn't change anything but uses the modern Py_GetConstant() instead of borrowed reference to Py_None.

[codspeed-hq]

CodSpeed Performance Report

Merging #1182 will improve performances by 23.94%

_{Comparing no-borrowed-refs (d719d96) with master (76abaac)}

Summary

⚡ 4 improvements
✅ 240 untouched benchmarks

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
⚡	test_cimultidict_getall_istr_hit[pure-python-module]	120.9 µs	97.7 µs	+23.78%
⚡	test_cimultidict_getall_istr_miss[pure-python-module]	42.2 µs	35.1 µs	+20.21%
⚡	test_multidict_getall_str_hit[case-insensitive-pure-python-module]	119.9 µs	96.7 µs	+23.94%
⚡	test_multidict_getall_str_miss[case-insensitive-pure-python-module]	40.9 µs	34 µs	+20.36%