AritistList proxies copy contents on iteration

When iterating over the contents of the the dynamically generated proxy lists

for the Artist-type accessors (see :ref:`Behavioural API Changes 3.5 - Axes

children combined`), 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. In this case

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 to affect the iteration over any of the other accessors, 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.