PR 1883 cleans up the code related to calling type slots.

* Use _PyObject_LookupSpecial() instead of lookup_maybe() in set_names(). This isn't performance critical.
* Inline lookup_maybe() in _PyObject_LookupSpecial(). This was the only remaining use of lookup_maybe().
* Replace lookup_maybe_method() and following raising an exception with lookup_method() in call_method().
* Replace lookup_method() with lookup_maybe_method() if the exception is ignored.
* Update outdated comments.
* Use call_method() in slot_sq_item. The comment about super-optimized version was outdated, now call_method() implements this.