Switch handlers to asynchronous generators

[willow-bohlen]

I was just looking around in the code, and noticed this at line 88 of cactusbot/handler.py:

# TODO: In Python 3.6, with asynchronous generators: # yield packet

Well we're in 3.6 now, so go for it.

[Innectic]

This can't be done yet. We can't assume that everyone is always on 3.6, because it's too new. This is being pushed to 0.5.

[willow-bohlen]

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[Innectic]

Merged into #254