[issue25609] Add a ContextManager ABC and type

[Guido van Rossum]

Guido van Rossum added the comment:

The abstract base class would be an ABC providing documentation and a template for implementations, and a way to test using isinstance(). This is how several other ABCs are used that have no particularly useful default implementation.

In addition, the presence of the ABC provides a reference for corresponding class in the typing module (used to express type hints using PEP 484) -- again this is just following the example of the other ABCs.

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<http://bugs.python.org/issue25609>
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