The 'msgpack-python' project on PyPI is now deprecated [1] in favor of the 'msgpack' one [2].
This simple change should fix recent CI builds of `swh-scheduler` [3] as `celery` 4.3.0
has just been released and it now depends on `msgpack` instead of `msgpack-python`.
[1] https://pypi.org/project/msgpack-python/
[2] https://pypi.org/project/msgpack/
[3] https://jenkins.softwareheritage.org/job/DSCH/job/tests/306/console