pythonPackages.cassandra-driver: 3.15.1 -> 3.16.0 - fix build

cassandra-driver requires an older version of cython than the one
present in nixpkgs.

Next cassandra-driver version will support cython 0.29
https://github.com/datastax/python-driver/commit/82c84255ff463998f31e11f0db81e18aad0f08df