The coverage pip package is not included in the dependencies

[markkuriekkinen]

The package setup script (https://github.com/Aalto-LeTech/python-grader-utils/blob/master/setup.py) lists the dependencies (pip packages) that are used by python-grader-utils. However, the coverage.py package is missing even though it is used by testcoveragemeta.py: https://github.com/Aalto-LeTech/python-grader-utils/blob/e5eb6d6de0465e4eb45ebda8cbb2dcb5449b3d24/graderutils/testcoveragemeta.py#L33

Our apluslms/grade-python container installs the coverage package in a separate layer "y2" (https://github.com/apluslms/grade-python/blob/6f01c6586cdbeee36d63fa521e12ca0feac53cf0/Dockerfile.y2#L5), but it should always be installed with python-grader-utils as a dependency since python-grader-utils uses the coverage package.

[raphendyr]

Ther requirement can also be added as an extras. The fetaure is not core of the library, but the library provides utilities to help with coverage stuff. Thus, this could make sense and might be the reason why it's missing from the settings.py.

The container (or a layer) can then use pip install 'graderutils[coverage] >= 3.0' to also install coverage requirements.