Implement Multi-Phase Init for the _testinternalcapi Module

[ericsnowcurrently]

See PEP 630 and PEP 687. This is an internal test module so the bar isn't as high as for regular stdlib modules.

Linked PRs

• GH-100997: Implement Multi-Phase Init for the _testinternalcapi Module #100998
• GH-100997: fix refleak in _testinternalcapi.c #101058

[kumaraditya303]

With #100998 there are refleaks on main, reopening.

[ericsnowcurrently]

The leak is fixed now, right?

[kumaraditya303]

Yeah fixed by #101058