TestPlotMatplotlib tests fail in oldestdeps CI cron run

[yarikoptic]

see https://github.com/con/duct/actions/runs/20634069981/job/59256486111

=========================== short test summary info ============================
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_sanity - D...
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_uses_agg_backend_with_output
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_file_not_found
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_invalid_json
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_info_json
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_info_json_absolute_path
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_malformed_usage_file
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_interactive_backend_with_get_backend
FAILED test/test_plot.py::TestPlotMatplotlib::test_matplotlib_plot_no_backend_registry
================== 9 failed, 318 passed, 2 skipped in 49.43s ===================

likely all those are the same