Consider using asyncio.run implementation in AsyncToSync

[andriilahuta]

Python 3.7 introduces new asyncio.run function. Would be nice to backport its implementation for standalone loop in AsyncToSync. It adds an extra level of reliability by wrapping loop.run_until_complete with try/finally, cancelling pending tasks and closing async generators.
On a side note, it seems that loop.create_task should be preferred to asyncio.ensure_future in most cases, according to Guido.

[andrewgodwin]

When Python 3.7 is released I'll come back and consider it, but we're still months away and we'd have to keep the existing implementation anyway to support 3.5. I will, however, go and crib from the implementation to make sure AsyncToSync is doing all the right cleanup.

As for create_task, I've moved to that here: c7e0065