Run pytest-mpl tests in CI

Quantify-scheduler had tests marked with @pytest.mark.mpl_image_compare, but these were not being run as the dependency was not installed and the test job does not use the required --mpl option.

In my circuit diagram refactor MR (quantify-scheduler!1071 (merged)), I added this dependency. This CI MR is needed to run the mpl tests in the CI pipeline.