bpo-40549: posixmodule.c uses defining_class (GH-20075)

Pass PEP 573 defining_class to os.DirEntry methods. The module state
is now retrieve from defining_class rather than Py_TYPE(self), to
support subclasses (even if DirEntry doesn't support subclasses yet).

* Pass the module rather than defining_class to DirEntry_fetch_stat().
* Only get the module state once in _posix_clear(),
  _posix_traverse() and _posixmodule_exec().

Author: vstinner