Switch from redis-py-cluster to redis-py

As documented, redis-py-cluster has reached EOL. It is now being merged into redis-py. We previously tried to switch from redis-py-cluster to redis-py but ran into various issues; see #2328. The upstream bug has since been fixed, so we can now switch from redis-py-cluster to redis-py.

Related, we should try to get redis-cluster integration tests running using testcontainers somehow, and part of the normal configurations tested.