Add deprecation warning for dropping Sphinx 2 and Python 2 support

One of the upcoming releases will drop Python 2 support, by bumping to Sphinx 2+ dependency. We should probably start warning users of this. This seems like something we can drop into a `1.0` release here. Dropping sphinx <1.6 support was merged already, so that will be our 1.0 release, along with the deprecation warning about html4 writer.