'msgpack-python' is deprecated, use 'msgpack' instead

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 [4].

• [1] https://pypi.org/project/msgpack-python/
• [2] https://pypi.org/project/msgpack/
• [3] https://jenkins.softwareheritage.org/job/DSCH/job/tests/306/console
• [4] http://docs.celeryproject.org/en/latest/whatsnew-4.3.html#messagepack-serializer

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Migrated from D1331 (view on Phabricator)