Docs don't build under Python 3

[jvkersch]

There are some Python 2-isms in conf.py which are easily fixed, but some of the Sphinx plugins seem like more work (e.g. CommentEater uses the compiler module that was removed under Python 3).

[jvkersch]

Probably what we want to do here is move to a set of external Sphinx plugins, like described in #384 (review)