Use critical sections in _warnings.c rather than mutex #160397

Feature or enhancement

Some advantages of critical sections:

• when running this C code under the GIL-enabled build, no locking is required. A critical section is automatically a no-op there.
• with a refactor, the regions holding the critical sections will be smaller and shorter. This improves performance and reduces chances of deadlocking.
• we don't need a reset of the warnings mutex after fork (as done in GH-158974)
• we don't have the issue of the mutex being held in daemon threads blocking shutdown (see GH-159485)

Has this already been discussed elsewhere?

This is a minor feature, which does not need previous discussion elsewhere

Links to previous discussion of this feature:

No response

Linked PRs

• gh-160397: Don't take the warnings lock when emitting a warning #160401