To be consistent you'd have to do the attribute check in PyObject_Call as well, i.e. if an object isn't callable, then trying to call it should raise a TypeError. I think the cost can be mitigated by only checking heap types (i.e. tp_flags & Py_TPFLAGS_HEAPTYPE).

It would be cleaner if slot_tp_call failed by raising TypeError instead of letting the AttributeError bubble up. There's a precedent in slot_tp_iter to raise a TypeError if lookup_method() fails. This would avoid having to change PyObject_Call.