Sorry, realized that my pure Python algorithm isn't equivalent to _PyType_Lookup() --- it fails the staticmethod example of Serhiy.  A closer one would be:

    for t in type(a).__mro__:
        if '__index__' in t.__dict__:
            return t.__dict__['__index__'].__get__(a)()

But it's still not a perfect match.  I think right now the only "perfect" answer is given by workarounds like "range(a).stop"...