`sage.graphs`: Eliminate use of `sage.plot.colors` when not plotting

[mkoeppe]

Currently, at least sage.graphs.graph_coloring uses it outside of a plotting context, which makes packaging (#35095) awkward

@dcoudert

[dcoudert]

We use method rainbow in sage.graphs.graph_coloring. It's useful here to assign colors to vertices or edges that may later be plotted.

[mkoeppe]

Yes, I saw that. Would it be possible to defer this assignment somehow?

[dcoudert]

Removing calls to rainbow from sage.graphs.graph_coloring is not easy. It would require to get surrounding methods in (at least) graph.py to assign colors.

I will check the feasibility of making methods in sage.graphs.graph_coloring return color classes only (or a number for counting) and to add a method in charge of turning the color classes to the desired format: color classes (do nothing), mapping vertex to color, mapping color to vertices. This way, calls to rainbow would be contained in a single method.