[docs] [issue37913] Document that __length_hint__ may return NotImplemented

[Steve Dower]

Steve Dower <steve.dower at python.org> added the comment:


New changeset aa3b629a09bfcdf26cdfbbf7d6445ae333c0d44d by Steve Dower in branch '3.7':
bpo-37913: document that __length_hint__ can return NotImplemented (GH-15383)
https://github.com/python/cpython/commit/aa3b629a09bfcdf26cdfbbf7d6445ae333c0d44d


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