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>