Add dependency on setproctitle

https://github.com/rq/rq/blob/cd17d17d71425cb75e887f3815cb5502d9a613e8/rq/worker.py#L50-L54

RQ workers use an optional dependency, setproctitle to give readable names to processes created for RQ. This could help when troubleshooting RQ jobs running in production, as there are no other ways to tell which PID matches a specific RQ job.