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I like this API too, and the patch looks good
apart from some more details:

A style note first: some lines are indented with 
spaces instead of tabs.  This causes some
changes on otherwise-unchanged lines, too.

if PyIndex_Check(key)   =>   if (PyIndex_Check(key))

typeobject.c: slot_nb_index() may not need to do 
the type-checking because it is done anyway in 
the caller, which can only be PyNumber_Index().

classobject.c: same remark about instance_index().

unicodeobject.c: kill macro HAS_INDEX
mmapmodule.c:    idem
arraymodule.c:   idem

should operator.index(o) return 
PyNumber_AsSsize_t(o) or just PyNumber_Index(o)? 
I can think of use cases for the latter, which
is somehow the most primitive of the two, so it
should IMHO be exposed via the operator module.

docs: "possitive" => "positive"