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The PyNumber_Index docs should explicitly state that it
raises IndexError when overflow occurs.

It may also be worth resurrecting _PyLong_AsClippedSsize_t
in order to clean up the implementation of
PyNumber_AsSsize_t (detecting OverflowError then poking
around in the guts of a long object is a bit ugly).

Other than that, looks good to me (I like this API a lot
better than mine).