[issue25609] Add a ContextManager ABC and type

[Guido van Rossum]

Guido van Rossum added the comment:

A slight problem is that I'd really like to keep the source of typing.py identical in Python 3.5 and 3.6. So maybe the definition should be conditional on the presence of AbstractContextManager?

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue25609>
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