Check for pytest tests packages dependencies at configuration time

[xvallspl]

Right now we have the dependencies in requirements.txt. Wouldn't it be nicer to raise an error/warning at configuration time if the requirements are not found and invite to install them beforehand?

[eguiraud]

would #123 fix this?

[xvallspl]

Not as it's written, because you would still need the other libraries: pytest-csv, etc...but yeah, they could be merged into a single issue

[eguiraud]

@stwunsch is the intent of #123 to also fail at configure time if any of the required python packages is missing?