installation: unpin urllib3
installation: unpin urllib3

* Unpins urllib3 since it is correctly set as a dependency in the
  Python Kubernetes client (addresses reanahub/reana-client#320).

reanahub/reana-job-controller,tiborsimko/reana-job-controller,diegodelemos/reana-job-controller,tiborsimko/reana-job-controller,reanahub/reana-job-controller,diegodelemos/reana-job-controller