journal: Allow to use backports when needed
At the moment, we need msgpack's backport version:
The following packages have unmet dependencies:
python3-swh.journal : Depends: python3-swh.core (>= 0.0.55) but 0.0.54-1~swh2~bpo9+1 is to be installed
Depends: python3-msgpack (>= 0.5.1) but 0.4.8-1 is to be installed