Use gevent as gunicorn backend + dep upgrades

I recently DOSed httpbin.org accidentally by having a test script running concurrently against /delay/:n which blocked all available workers. Using gevent is probably the easiest way to work around that.

I also took the liberty and upgraded a few dependencies. Besides, there was a broken import which I believe came from b387ee2deab33d284253b6a81fe643f664639188.

The downside of using gevent is that we can't use pypy at the moment. :(