async is a keyword on 3.7

[hroncok]

Fixes #44

[ciscorn]

@hroncok It should be something like:

ensure_future = getattr(asyncio, 'ensure_future', None) or getattr(asyncio, 'async')

otherwise the following error occurs on Python 3.7:

AttributeError: module 'asyncio' has no attribute 'async'

[0bsearch]

I propose to drop None in last getattr, to make it fail eventually if neither async/ensure is available.