Add pypy support to tooling and documentation

Did you know hyper *already* supports PyPy? It's true! All that was needed was this (clearly herculean) effort to add it to tox, Travis, and the docs, and boom, PyPy.

P.S. For some reason, the PyPy tests take upwards of six minutes on Travis CI - maybe we need to disable parallelism? I think there's an env var you can check...