Issue #25150: Hide the private _Py_atomic_xxx symbols from the public

Python.h header to fix a compilation error with OpenMP. PyThreadState_GET()
becomes an alias to PyThreadState_Get() to avoid ABI incompatibilies.

It is important that the _PyThreadState_Current variable is always accessed
with the same implementation of pyatomic.h. Use the PyThreadState_Get()
function so extension modules will all reuse the same implementation.