Add Debian package python3-cassandra

https://pypi.org/project/cassandra-driver/ is missing from buster (it's in stretch and bullseye though: https://packages.debian.org/bullseye/python3-cassandra)

It will be needed to build swh-storage's package after !316 (closed) lands.

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Migrated from T2243 (view on Phabricator)