Update pytest-mpl configuration to use the default matplotlib style

It reverted the styling to "classic", probably to prevent massive churn
in projects using pytest-mpl when matplotlib changed its defaults.
However, the default cmap in "classic" mode is "jet" - the colormap we
are trying to avoid!

Author: mx-moth

pyproject.toml

@@ -101,6 +101,7 @@ markers = [
 
 mpl-use-full-test-name = true
 mpl-baseline-path = "tests/baseline_images"
+mpl-default-style = "default"
 
 [tool.mypy]
 python_version = "3.12"