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

[Jeroen Demeyer]

Change by Jeroen Demeyer <jeroen.k.demeyer at gmail.com>:


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keywords: +patch
pull_requests: +15093
stage:  -> patch review
pull_request: https://github.com/python/cpython/pull/15383

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37913>
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