l substitutions performed.

For unions, basic simplifications used by Union constructor are performed.
Examples::
    get_args(Dict[str, int]) == (str, int)
    get_args(int) == ()
    get_args(Union[int, Union[T, int], str][int]) == (int, str)
    get_args(Union[int, Tuple[T, int]][str]) == (int, Tuple[str, int])
    get_args(Callable[[], T][int]) == ([], int)
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