Builds fail with pytest 4.1

[shashank88]

Looking at: pytest-dev/pytest#4546
this breaks our builds as the error iirc reflected missing get_marker.

[shashank88]

pytest-dev/pytest-cov#253 should fix it once they release.

[shashank88]

We can lock pytest to the previous successful version (4.0.2 from travis) if the release is not done by tomorrow.

[shashank88]

Pytest-cov updated, will revert this and pin pytest-cov >=2.6.1