installation: harmonise pytest versions
installation: harmonise pytest versions

* Closes reanahub/pytest-reana#41

Signed-off-by: Jan Okraska <57f2532bb355b938245ee70fb8b246c980a8dd1b@cern.ch>

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