Fix python packaging violations detected by taskotron
    
    kobo-admin was packaged for both python2 and python3.
    According to packaging guidelines, we should prefer to package only the
    python3 version.
    
    python3-kobo-rpmlib was wrongly depending on python2 RPM bindings.
    
    python2-kobo-django and python2-kobo-rpmlib should depend on python2-*
    packages where possible (e.g. prefer Requires: python2-django over
    python-django).