Close #19030: improvements to inspect and Enum.

inspect.getmembers and inspect.classify_class_attrs now search the metaclass
mro for types.DynamicClassAttributes (what use to be called
enum._RouteClassAttributeToGetattr); in part this means that these two
functions no longer rely solely on dir().

Besides now returning more accurate information, these improvements also
allow a more helpful help() on Enum classes.