ate final methods and final classes.

Use this decorator to indicate to type checkers that the decorated
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
attempts to set the ``__final__`` attribute to ``True`` on the decorated
object to allow runtime introspection.
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