Links broken in inheritance diagram due to change in class paths

As a result of #1826, links in the inheritance diagrams no longer work. For example, try clicking on a class in the diagram at:

http://docs.astropy.org/en/stable/time/index.html

then try in:

http://docs.astropy.org/en/latest/time/index.html

cc @karbary @eteq 

This is causing a lot of warnings when using the Sphinx ``nitpicky=True`` option, hence how I noticed it.

(Personally, I never use the diagrams, and would actually be in favor of turning them off)