Issue #18408: slot_tp_str() must not fallback on slot_tp_repr() on error

type->tp_str must not point to slot_tp_str() if type has no __str__ attribute,
so there is no reason for slot_tp_str() to fallback on slot_tp_str() on lookup
error. Moreover, calling PyErr_Clear() may hide a real bug like MemoryError.

If __str__ attribute is removed, slots must be updated (which is done by
type_setattro()).