[SIM907] Use Optional[Type] instead of Union[Type, None] about flake8-simplify HOT 3 CLOSED

$ astpretty --no-show-offsets /dev/stdin <<< `cat example.py`
Module(
    body=[
        FunctionDef(
            name='foo',
            args=arguments(
                posonlyargs=[],
                args=[
                    arg(
                        arg='a',
                        annotation=Subscript(
                            value=Name(id='Union', ctx=Load()),
                            slice=Tuple(
                                elts=[
                                    Name(id='int', ctx=Load()),
                                    Constant(value=None, kind=None),
                                ],
                                ctx=Load(),
                            ),
                            ctx=Load(),
                        ),
                        type_comment=None,
                    ),
                ],
                vararg=None,
                kwonlyargs=[],
                kw_defaults=[],
                kwarg=None,
                defaults=[],
            ),
            body=[
                Return(
                    value=Name(id='a', ctx=Load()),
                ),
            ],
            decorator_list=[],
            returns=Subscript(
                value=Name(id='Union', ctx=Load()),
                slice=Tuple(
                    elts=[
                        Name(id='int', ctx=Load()),
                        Constant(value=None, kind=None),
                    ],
                    ctx=Load(),
                ),
                ctx=Load(),
            ),
            type_comment=None,
        ),
    ],
    type_ignores=[],
)