ArtistList proxies copy contents on iteration#

When iterating over the contents of the the dynamically generated proxy lists for the Artist-type accessors (see Axes children are no longer separated by type), a copy of the contents is made. This ensure that artists can safely be added or removed from the Axes while iterating over their children.

This is a departure from the expected behavior of mutable iterable data types in Python -- iterating over list while mutating it has surprising consequences and dictionaries will error if they change size during iteration. Because all of the accessors are filtered views of the same underlying list, it is possible for seemingly unrelated changes, such as removing a Line, to affect the iteration over any of the other accessors. In this case, we have opted to make a copy of the relevant children before yielding them to the user.

This change is also consistent with our plan to make these accessors immutable in Matplotlib 3.7.