Serialization model for single and double tee pieces should be unified.

[zuckerruebe]

As a developer I'd like to unify the serialization classes for single and double pipe tee pieces to have less classes to maintain. This requires updating the serialization framework to allow for a set of superseded classes in

    @classmethod
    @_abc.abstractmethod
    def getSupersededClass(cls) -> _tp.Type[UpgradableJsonSchemaMixinVersion0]:
        raise NotImplementedError()

i.e.

    import collections.abc as _cabc
    # ...
    @classmethod
    @_abc.abstractmethod
    def getSupersededClass(cls) -> _cabc.Set[_tp.Type[UpgradableJsonSchemaMixinVersion0]]:
        raise NotImplementedError()