bpo-38631: Avoid Py_FatalError() in init_slotdefs() (GH-18263)

Rename init_slotdefs() to _PyTypes_InitSlotDefs() and add a return
value of type PyStatus. The function is now called exactly once from
_PyTypes_Init(). Replace calls to init_slotdefs() with an assertion
checking that slotdefs is initialized.