Should ArtistList implement __add__? #19890

Problem

ArtistList (#18216) is great :) but I realized that I had a few places in my old programs where I was doing e.g. ax.lines + ax.images + ..., which now breaks because there's no ArtistList.__add__. Instead one can do [*ax.lines, *ax.images, ...] but perhaps implementing ArtistList.__add__ may help with the transition?

Proposed Solution

Implement ArtistList.__add__ (which should only add ArtistLists and/or lists, and return plain lists). ArtistList.__iadd__ should probably also be implemented as throwing an exception (or emitting a deprecation warning), as direct modification of ArtistLists is currently being deprecated.

Additional context and prior art

N/A