Use python-cassandra 3.21

swh-storage can currently use only python-cassandra 3.19 and 3.20 because earlier versions didn't yet support Cassandra 4.0, and version 3.21 doesn't either because of a bug.

However, we want to use version 3.21 because it's the first one to provide wheels on PyPI, and build time without wheels is very long.

https://datastax-oss.atlassian.net/browse/PYTHON-1205

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Migrated from T2266 (view on Phabricator)