Why does 'tf2' have a dependency on rospy?

https://github.com/ros/geometry_experimental/blob/hydro-devel/tf2/package.xml#L24

@tfoote I see that tf2 is now a standalone library.  Is there a reason that we need to have rospy as a dependency for the C++ library package?  The only python I see here is for Sphinx.

https://github.com/ros/geometry_experimental/blob/hydro-devel/tf2/conf.py