bpo-43162: [Enum] deprecate enum member.member access (GH-24486)

In 3.5 (?) a speed optimization made it possible to access members as
attributes of other members, i.e. ``Color.RED.BLUE``.  This was always
discouraged in the docs, and other recent optimizations has made that
one no longer necessary.  Because some may be relying on it anyway, it
is being deprecated in 3.10, and will be removed in 3.11.