Not all test dependencies covered

[BenjaminBossan]

When doing a pip install .[tests], not all dependencies required to run the tests are installed. Concretely, some tests require pandas but pandas is only a docs dependency. Maybe there are others, I haven't checked.

Possible solutions are:

1. Make it possible for dependent_packages to have multiple entries, so that they can be required for both tests and docs.
2. Rewrite tests so that they are skipped if their dependency is not covered (e.g. pd = pytest.importorskip("pandas")).