Summary

This is a collection of ideas to make more efficient use of CI resources. Benefits are:

• Less cost: While we don't pay for CI, our CI donors implicitly bear the cost of runnig our jobs. We should hande the resources that we are given responsibly
• Less environmental impact
• Possibly faster cycle times

Proposed fix

This is a collection of possible ideas:

1. Speed up the test suite:
   1.1. Make our code faster. To be checked, but I'd guess that figure creation and drawing are the expensive parts.
   1.2. Optimize test types.
   - Cost of the test types: @check_figures_equal() (2 drawn figures) > @image_comparison (1 drawn figure) > just checking that proper change have been made to Axes / Artists (0 drawn figures) - e.g. check that new artists have been created, artist has the right color etc.
2. Revisit the test matrix. Do we need all combinations?
3. Two-stage tests, e.g. if test_*.py files are modified, do a pre-run only on these files. If they break, stop. If they pass, do the full test run.
4. Don't run tests on draft PRs.
5. Add a label skip-tests for manual control.
   ...