Return a list of the superclasses of the given class

def _list_superclasses(class_def):
    """Return a list of the superclasses of the given class"""
    superclasses = class_def.get('superClasses', [])
    if superclasses:
        # Make sure to duplicate the list
        return list(superclasses)

    sup = class_def.get('superClass', None)
    if sup:
        return [sup]
    else:
        return []