Use compatible pytest,pytest-xdist,pytest-cov (#321)
Use compatible pytest,pytest-xdist,pytest-cov (#321)

Travis py36 and py37 failed because it installed incompatible
pytest(5.4.3) and pytest-xdist(2.0.0).
https://travis-ci.com/github/emc-openstack/storops/jobs/373116173, line
251, 255 and 296.

This commit just limit the version of pytest, pytest-xdist and
pytest-cov for py36 and later.
emc-openstack/storops,emc-openstack/storops