rated method cannot be overridden, and decorated class
        cannot be subclassed. For example:

            class Base:
                @final
                def done(self) -> None:
                    ...
            class Sub(Base):
                def done(self) -> None:  # Error reported by type checker
                    ...
            @final
            class Leaf:
                ...
            class Other(Leaf):  # Error reported by type checker
                ...

        There is no runtime checking of these properties. The decorator
        sets the ``__final__`` attribute to ``True`` on the decorated object
        to allow runtime introspection.
        T)