Applied patch.  In theory, this was reasonable.  Since islice() deals with indices, checking __index__ seems reasonable.

On the other hand, stable high-performance code was changed without any known reasonable use cases, and no one will ever likely see a benefit. A cheap atomic operation was replaced with a more complex and less thread-safe chain of calls (I'm no longer even sure that the test cases cover all the possible code paths).

Users might be better off by not seeing an unhelpful error message when passing in a numpy.int32, or they might be worse-off by having lost a cue that they were writing inefficient code (which invisibly creates temporary integer objects on every call when presumably the whole reason for using numpy was a concern for efficiency).