Replace asyncore with asyncio

asyncore is AFAIK mostly dead at this point and being replaced by asyncio. It's not only hip and cool but also offers a higher level of abstraction and would make it easier to integrate with other tools.

This would mean dropping python2 (again ; )

I pushed back to Python2 to test out PyPy for performance reasons. It still hadn't been enough :( . Feel free to get rid of the six crap if we're leaving Python2 behind again.

There is a pypy3 these days I think

Yeah, but it only supports 3.2 or something.