Coverage is incomplete due to some tests running in sub-processes

Our coverage is actually better than what is reported by the coverage plug-in - the issue is because e.g. sphinx is being run in a subprocess, the coverage isn't being reported for a lot of the sphinx code. It would be nice to find a way to run sphinx without instantiating a subprocess in order to better see which code we cover.
I felt like this *should* be possible, but I think there's some reason we generally don't actually run sphinx this way for the Astropy docs.  That said, for the present purposes it seems fine.