Remove pyfits depency

[johndonor3]

Installing on a new machine illustrates how very abandoned this package is. It should be trivial to replace with astropy (yes that's me volunteering)

[albireox]

Great! It is not such a totally trivial change though. astropy.fits and pyfits have some subtle differences (assuming you're considering one of the latest astropy versions, which you should). Fortunately, those differences mostly matter when creating new FITS files, which STUI does not do.