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

[Raymond Hettinger]

Raymond Hettinger <raymond.hettinger at gmail.com> added the comment:

> Then I guess what I consider a bug is "__init__ is undocumented".

No, this just how Python works.

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<https://bugs.python.org/issue37145>
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