In some situations, where the Base class defines an Attribute, the
BaseMeta class doesn't properly see the _get_parent_attribute or
_get_attr_<whatever> methods because of multiple layers of subclasses
(ie. Handler, which subclasses Task). This addresses that by merging
the __dict__ of the parent with the current classes __dict__ meaning
all future iterations see available special methods.
Fixes#18378
4794b5da45