Running tests requires matplotlib

[rth]

As part of pytest migration in #527, it looks like matplotlib is hard dependency for running tests, despite what setup.py requirements, and CONTRIBUTING.md says.

It is imported in

openml-python/tests/test_examples/test_OpenMLDemo.py

Line 5 in 7a612ea

import matplotlib

but doesn't seem to be used (added in #430).

Is it required by nbformat?

An alternative could be to skip corresponding tests.

[mfeurer]

We used to have a jupyter notebook as an example which used matplotlib. In the last few weeks, we replaced it by multiple small python examples and removed the plotting for the moment. The notebook was unit tested, and I guess we can remove that unit test for now. Do you want to create a PR for this?

[PGijsbers]

With many issues open on the same topic, I decided to close this one. All further discussion can be had in #596.