Upgrade pytest and pytest-cov

[scop]

Description:

Upgrade pytest to 4.0.0: https://docs.pytest.org/en/latest/changelog.html#pytest-4-0-0-2018-11-13

...and pytest-cov to 2.6.0: https://pytest-cov.readthedocs.io/en/latest/changelog.html#id1

Checklist:

• The code change is tested and works locally.
• Local tests pass with tox. Your PR cannot be merged unless tests pass
• There is no commented out code in this PR.

[fabaff]

Thanks 🐦