PR Summary

Closes #23819

Clarify that it is enough to implement one of draw_gouraud_triangle and draw_gouraud_triangles in a backend and that any Artist should call the latter.

Remove unused versions of draw_gouraud_triangle.

I assume that there should be some sort of note here clarifying the changed behaviour (in practice that any custom artists should call draw_gouraud_triangles). Including the example code:

    def draw_gouraud_triangle(self, gc, points, colors, trans):
        self.draw_gouraud_triangles(gc, points.reshape((1, 3, 2)),
                                    colors.reshape((1, 3, 4)), trans)

I do not really see a way to deprecate this, without really deprecating draw_gouraud_triangle completely (and instead introduce something like _draw_gouraud_triangle which wouldn't really fit well with the general structure). It would be possible to deprecate the now removed versions though. A quick search show that https://github.com/ArduPilot/MAVProxy uses a copy of the backend_agg.py-file which has it included, but that is the only thing I could find which may cause anyone some problem.

PR Checklist

Tests and Styling

• [N/A] Has pytest style unit tests (and pytest passes).
• Is Flake 8 compliant (install flake8-docstrings and run flake8 --docstring-convention=all).

Documentation

• [N/A] New features are documented, with examples if plot related.
• [N/A] New features have an entry in doc/users/next_whats_new/ (follow instructions in README.rst there).
• API changes documented in doc/api/next_api_changes/ (follow instructions in README.rst there).
• Documentation is sphinx and numpydoc compliant (the docs should build without error).