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Hmm.  In my working copy, PySlice_GetIndicesEx now has a
different interface (it computes the length of the slice, as
that's fiddly and almost always required).  So I may remain
cowardly here.  As a bonus, I'll document both functions in
Doc/api/.

Yes, I was staring at my code and thinking "that can't get
all the end cases, can it?".  Almost a relief to hear it
doesn't :)  I'll get pen and paper out.