gh-149211: Preserve custom warning handler during finalization

[iamrajhans]

Fixes gh-149211.

This keeps the public warnings module reachable by the _warnings accelerator during late interpreter finalization via a weak reference, so warnings.warn() can still dispatch through a custom warnings.showwarning after sys.modules cleanup.

Tests run:

• make -j8
• ./python.exe -m test test_warnings -m test_showwarning_during_finalization -v
• ./python.exe -m test test_warnings
• git diff --check