pythongen may inject additional unknown_ attributes into __post_init__ when type designators are used

[cmungall]

When type_designators are used on a slot s, pythongen can inject an additional attribute unknown_s into the post_init. This generally doesn't cause issues but can be confusing if an object is introspected with vars.

This can be seen on the current compliance tests for type_designator. The test passes, but can be seen in the generated class: tests/test_compliance/output/test_designates_type/Rstring/generated/python_dataclasses.py:

    def __post_init__(self, *_: List[str], **kwargs: Dict[str, Any]):
        self.type = str(self.class_name)

        super().__post_init__(**kwargs)
        self.unknown_type = str(self.class_name)


    def __new__(cls, *args, **kwargs):

        type_designator = "type"
        if not type_designator in kwargs:
            return super().__new__(cls,*args,**kwargs)
        else:
            type_designator_value = kwargs[type_designator]
            target_cls = cls._class_for("class_name", type_designator_value)


            if target_cls is None:
                raise ValueError(f"Wrong type designator value: class {cls.__name__} "
                                 f"has no subclass with ['class_name']='{kwargs[type_designator]}'")
            return super().__new__(target_cls,*args,**kwargs)