test_plotter.py::test_matplotlib_plot fails on matplotlib-3.4.2

Plotting itself seems to work in general, but specified test fails with the error:

======================================================================== short test summary info =========================================================================
FAILED kwant/tests/test_plotter.py::test_matplotlib_plot - AttributeError: 'Path3DCollection' object has no attribute '_z_markers_idx'

There are several matplotlib-related warnings, they might point to a reason.

pytest kwant/tests/test_plotter.py::test_matplotlib_plot output

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