bpo-40170: PyObject_GET_WEAKREFS_LISTPTR() becomes a function (GH-19377)

Convert the PyObject_GET_WEAKREFS_LISTPTR() macro to a function to
hide implementation details: the macro accessed directly to the
PyTypeObject.tp_weaklistoffset member.

Add _PyObject_GET_WEAKREFS_LISTPTR() static inline function to the
internal C API.