Fix python3.6 compatibility

[fipwmaqzufheoxq92ebc]

The library currently uses methods like get_running_loop(), which are only available in python>=3.7
This PR fixes that.

Fixes the downstream-issue flyte/mqtt-io#192

[flyte]

Nice one @fipwmaqzufheoxq92ebc ! Looks good to me, but I'll hang on until @frederikaalund has a look 😃

[frederikaalund]

Thanks for this pull request. It looks good to me. 👍

Do note that asyncio-mqtt officially targets Python 3.7. However, in the spirit of pragmatism:

• We'll gladly merge in small compatibility fixes like this (thanks for the contribution 👍)
• We'll try not to break backwards compatibility (this is on a best-effort basis).