Support Python 3.7

[anders-was-here]

In Python 3.7 async is a reserved word. See: https://docs.python.org/3.4/library/asyncio-task.html#asyncio.async

I needed to replace half a dozen instances of 'asyncio.async' with 'asyncio.ensure_future' to make it work.

Like:

-        asyncio.async(self._get_and_publish_event_log(), loop=self._event_loop)
+        asyncio.ensure_future(self._get_and_publish_event_log(), loop=self._event_loop)

[msabramo]

Hmmm, I just ran Python 3.7 tests in tox and it passed. Also, I didn't find any instance of async in the code on master. Can you clarify?

[anders-was-here]

Either it's fixed for it was posted to the wrong Github project.