test test_visualize/test_visualize_common fails after matplotlib upgrade

[Manangka]

In GitLab by @jdelsman on Apr 9, 2021, 16:50

Since matplotlib 3.4, the number of levels passed to BoundaryNorm must exceed 1 (matplotlib/matplotlib#17830)
This breaks two tests, that pass levels = [0.5] to BoundaryNorm.

[Manangka]

In GitLab by @jdelsman on Apr 12, 2021, 14:44

mentioned in commit 1361b0d