Summary

All's in the title. See e.g. https://github.com/matplotlib/matplotlib/runs/5056485756?check_suite_focus=true#step:12:184 ("SKIPPED [2] lib/matplotlib/tests/test_backends_interactive.py:159: Skipping {'MPLBACKEND': 'gtk4agg'} because no usable GTK bindings"), also check codecov at https://app.codecov.io/gh/matplotlib/matplotlib/blob/447160e0c78a3f834696fbf6b4dca967d9bd05a4/lib/matplotlib/backends/backend_gtk4.py.

Proposed fix

Install whatever dependencies are needed to run gtk4 on CI.