PR Summary

The long term plan is that all test images should use style='mpl20' (= style='default'). To be able to change the default of image_comparison` to this, it makes sense to change all tests that doesn't explicitly specify style to use the current default.

This PR handles mpl_toolkits.

The comment isn't that explicit as I expect that once the default is changed, one can simply reword it to drop the style. (I do not expect many regenerated images before that happens...)

PR Checklist

Documentation and Tests

• Has pytest style unit tests (and pytest passes)
• Documentation is sphinx and numpydoc compliant (the docs should build without error).
• New plotting related features are documented with examples.

Release Notes

• New features are marked with a .. versionadded:: directive in the docstring and documented in doc/users/next_whats_new/
• API changes are marked with a .. versionchanged:: directive in the docstring and documented in doc/api/next_api_changes/
• Release notes conform with instructions in next_whats_new/README.rst or next_api_changes/README.rst