fix package dependencies: add python3-dnf, drop pygtk2 and gtk3
    
    The app explicitly both runs 'dnf' commands and uses the DNF
    module, so expressing both dependencies seems a good idea.
    The explicit 'gtk3' dep is not needed, pygobject3 necessarily
    implies it and the app does not use GTK+ directly, only through
    pygobject3. The app has no dep on pygtk2 any more, this was
    entirely bogus.