[issue29679] Add @contextlib.asynccontextmanager

[Yury Selivanov]

Yury Selivanov added the comment:

> so it would need to split any code using "async def" and "await" out to a separate file that only gets imported on 3.5+ (and similarly only run the corresponding test cases on 3.5+).

This seems doable.  I'd only vote to keep contextlib.py as one file in CPython though.

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<http://bugs.python.org/issue29679>
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