[docs] [issue34077] doc Be explicit about mock_open created mocks not supporting __iter__

[Andrés Delfino]

Change by Andrés Delfino <adelfino at gmail.com>:


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keywords: +patch
pull_requests: +7753
stage:  -> patch review

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue34077>
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