Update cassandra-driver to 3.21.0. (#4636)
Update cassandra-driver to 3.21.0. (#4636)

This provides binary wheel files and reduces Docker build times drastically.
denisov-vlad/redash,alexanderlz/redash,alexanderlz/redash,getredash/redash,denisov-vlad/redash,denisov-vlad/redash,getredash/redash,alexanderlz/redash,denisov-vlad/redash,denisov-vlad/redash,getredash/redash,getredash/redash,getredash/redash,alexanderlz/redash