[issue25609] Add a ContextManager ABC and type

[Roundup Robot]

Roundup Robot added the comment:

New changeset 257dd57dd732 by Brett Cannon in branch '3.5':
Issue #25609: Backport typing.ContextManager.
https://hg.python.org/cpython/rev/257dd57dd732

New changeset 14cf0011c5e9 by Brett Cannon in branch 'default':
Merge for issue #25609
https://hg.python.org/cpython/rev/14cf0011c5e9

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