Update for newer RPM
    
    python < 4.14.2 returned bytes, but that is now changed to return
    decoded strings.
    
    Once Kobo is updated to work with newer RPM, these changes are still
    needed to get the test suite passing.
    
    Tox configuration is updated to run tests on multiple versions of
    python. It should now be sufficient to run it without installing any
    Python packages first.
    
    Relates: https://bugzilla.redhat.com/show_bug.cgi?id=1713107
    Relates: https://github.com/release-engineering/kobo/pull/115
    Signed-off-by: Lubomír Sedlář <lsedlar@redhat.com>