Use gstreamer >= 1.0, python 3 and asyncio

Completely overhauled pyamp to use more modern gubbins under the hood. Should have no functional change except for the fact that I fixed the fadeout bug (Issue #7) when I was fiddling with changing the controller code and began to understand more about how the controllers work.

I could just merge this on my machine, but part of the exercise here is to experiment with Github's workflows!