I guess it depends. If you are inheriting and overloading a class from Astropy, then yes you need to install Astropy. But in other cases (for several affiliated packages I have), all I use are the intersphinx links, and those get downloaded separately (from http://astropy.readthedocs.org/en/stable/objects.inv). In the first case, ``astropy`` can't be mocked anyway, so what I'm suggesting is only useful if the astropy imports in the package can be mocked.