Will not wrap slot functions from base classes if they are not explicitly overridden on class creations.

Subclass __getitem__() performance even increases a bit from:

$ ./python -m timeit -s $'class d(dict): pass\ni = d({1:2})' "i[1]"
10000000 loops, best of 5: 28.5 nsec per loop

to:

$ ./python -m timeit -s $'class d(dict): pass\ni = d({1:2})' "i[1]"
10000000 loops, best of 5: 21.6 nsec per loop

Rationale for the checks is as follows:

• generic == NULL because not sure of behavior with slots that loop several times like tp_getattr, tp_setattr and tp_richcompare.
• Py_IS_TYPE(descr, &PyMethodDescr_Type) because only if setting pre-existing builtin C method.
• *ptr == ((PyMethodDescrObject *)descr)->d_method->ml_meth checks both *ptr != NULL and stuff like:

class C:
    __new__ = type.__dict__['__subclasscheck__']

• Issue: Py_TYPE(lhs)->tp_as_mapping->mp_subscript is not the same as Dict_Type.tp_as_mapping->mp_subscript when it should be #132284