Register mpl_image_compare marker to remove PytestUnknownMarkWarning (#323)
Register mpl_image_compare marker to remove PytestUnknownMarkWarning (#323)

Supresses the annoying warning that @pytest.mark.mpl_image_compare emits because it isn't registered properly. See relevant pytest documentation at https://docs.pytest.org/en/latest/mark.html#raising-errors-on-unknown-marks
GenericMappingTools/gmt-python,GenericMappingTools/gmt-python