I added some debugging to the if(required) clause.  This is interesting:

basicsize 16
slotnames 0 basicsize 16
tp_basicsize 80 basicsize 16

tp_basicsize comes in at 16 bytes.  tp_dictoffset and tp_weaklistoffset must both be 0, and while slotnames is not Py_None, it must be adding 0 to the basicsize.  Since sizeof(PyObject *) can't be 0, it must mean that Py_SIZE(slotnames) is 0 (since they are multiplied).

But as you can see tp_basicsize is 80.  So essentially PyBaseObject_Type.tp_basicsize is 16 but obj->ob_type->tp_basicsize is 80, and that triggers the traceback.