Note that the docs for get_dashes was wrong: the dash list is in points,
not in pixels; this is documented e.g. in the line_demo_dash_control
example; also the default dashstyle is (None, None).

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code is Flake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way