gh-128384: Add locking to warnings.py.

[nascheme]

The warnings module has some (relatively minor) thread safety issues. The catch_warnings contex manager is a major issue but that will be handled in a different PR. There are races between updating the filters list and incrementing the _filters_version number. Also, the warn_explicit() uses the global state in a non-thread-safe way. These issues are relatively easy to fix with some extra locking.

Changes:

• expose the mutex used by the _warnings module, as _acquire_lock and _release_lock
• implement a lock context manager and lock module state as needed
• fix a bug in DeprecatedTests that resulted in mixing py_warnings and c_warnings modules.

The mutex used is non-reentrant and so some care is required to avoid deadlocks. I restructured the code in warn_explicit() to reduce functions called within the locked section. Perhaps a re-entrant lock should be used instead?

• Issue: warnings.catch_warnings is not thread safe or async safe #128384

[kumaraditya303]

The mutex used is non-reentrant and so some care is required to avoid deadlocks. I restructured the code in warn_explicit() to reduce functions called within the locked section. Perhaps a re-entrant lock should be used instead?

I think the lock should be re-entrant, as locks are held when doing dict lookups which can call arbitrary python code.

[kumaraditya303]

This looks good now from a brief look.