Run matplotlylib tests in CI

[robertoffmoura]

This is a follow up to #5697. It adds a dedicated test-matplotlylib job to run-pytest.yml that installs matplotlib and runs the tests across all supported Python versions (3.8–3.13)

Historically, the original matplotlylib tests (in tests/test_optional/test_matplotlylib/) were intentionally skipped in 2022 (b7ba00b) because they were failing. When the test_renderer suite (plotly/matplotlylib/tests/test_renderer.py) was added (#5312), it was unintentionally never wired into run-pytest.yml.

[camdecoster]

Thanks for putting this together! Could you please make a few minor changes?

[robertoffmoura]

Thanks! I applied the changes you suggested.

[camdecoster]

Okay, so one of the new tests is broken on Python 3.8 because it gets version 3.7.5 of matplotlib and that changes the results. _draw_line_collection produces a different result because of how ax.contour works in that version. We'll need to address that in another PR before merging this one. It didn't show up before because I hadn't merged #5770 yet. Any interest in tackling that?

[robertoffmoura]

I opened #5799 to address this. I think relaxing the test assertions on the older matplotlib version might be better than increasing the code complexity here, since the charts still look the same.

[camdecoster]

Sounds good. Python 3.8 is EOL anyway, so I don't think we need to worry about it too much.