[C API] Add `PyMutex_STATIC_INIT` macro

[vstinner]

Currently, self->mutex = (PyMutex){0}; or self->mutex = (PyMutex){_Py_UNLOCKED} pattern is used to initialize statically a PyMutex. It leaks implementation details of PyMutex.

I propose adding PyMutex_STATIC_INIT macro to abstract the initialization. The pattern becomes: self->mutex = PyMutex_STATIC_INIT;.

Linked PRs

• gh-142585: Add PyMutex_STATIC_INIT macro #142586