Replace asyncio.async with getattr in 2.3 bugfix release

[asvetlov]

The next aiohttp 3.0 will be not 100% backward compatible with aiohttp 2.3, we need to give our users a transition period.

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new