Fix compatibility with asyncio 3.4.1+

[asvetlov]

In new asyncio StreamWriter.drain has become coroutine instead of plain function.
That means that transport.drain() should be un-yielded.

Drawback is: in debug mode (PYTHONASYNCIODEBUG env var is on) CoroWrapper raises exception.

Will fix quickly and publish 0.9.3 release.
Need to setup travis for passing tests both with PYTHONASYNCIODEBUG and without it.

[asvetlov]

Fixed in 0.9.3

[fafhrd91]

I think we should stop using streams.py from asyncio.

[asvetlov]

@fafhrd91 maybe you are true but I'm not ready to drop streams right now.

[ludovic-gasc]

FYI, during PyCON-FR, I've discussed with some AsyncIO developers.
They asked me why in aiohttp the streams have been redefined.
Personally, I've no idea about the reason.

[lock]

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