* fix: Make gil_safe_call_once thread-safe in free-threaded CPython
The "is_initialized_" flags is not protected by the GIL in free-threaded
Python, so it needs to be an atomic field.
Fixes#5245
* style: pre-commit fixes
* Apply changes from code review
---------
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
