bpo-40170: Remove PyHeapType_GET_MEMBERS() macro (GH-30942)

Remove the PyHeapType_GET_MEMBERS() macro. It was exposed in the
public C API by mistake, it must only be used by Python internally.
Use the PyTypeObject.tp_members member instead.

Rename PyHeapType_GET_MEMBERS() to _PyHeapType_GET_MEMBERS() and move
it to the internal C API.