[docs] [issue37145] collections.abc.MappingView mixins rely on undocumented _mapping

[Karthikeyan Singaravelan]

Change by Karthikeyan Singaravelan <tir.karthi at gmail.com>:


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nosy: +rhettinger
versions:  -Python 3.5, Python 3.6

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Python tracker <report at bugs.python.org>
<https://bugs.python.org/issue37145>
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