Astropy page: let's use labels as a  source of truth for astropy-affiliated packages. (rather than the partners field)

Currently, we use .partners to find out if a package is astropy or partner affiliated. 
This is dangerous because any user could check the astropy partner box when submitting a package! but only an editor can declare a package astropy affiliated and add a label! 

This is a very small fix, but one that I think is worth doing to maintain the integrity of our peer review process. 