This may have been the most recent discussion of this idea (as far as I can tell):
http://mail.python.org/pipermail//python-ideas/2012-August/016036.html

Basically, it seems to be still unresolved in the trunk Python; sorry, I thought by now it would have been resolved e.g. by the addition of a method on types or a function in the operator module.  In the absence of either, you need either to simulate its behavior by doing this:

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

Or you can piggyback on an unrelated call that simply causes the C-level PyNumber_Index() to be called:

    return range(a).stop