Drop support for Python 3.5

[mehaase]

wsproto merged a change last month that drops support for Python 3.5: python-hyper/wsproto#116

This change hasn't been released yet, but it is planned to be part of their version 1.0 (tentatively scheduled for October 2019), so in order for us to consume wsproto==1.0, this project will need to drop Python 3.5 sometime around the end of 2019.

When we do this, we can close #54 and #84, and also remove the async_generator dependency.

I'm opening this thread now even though I'm not going to make any changes until wsproto forces us to. Please leave comments if you have concerns.