fix __match_args__ attribute name

[bzoracler]

>>> import ast
>>> m = ast.parse("type ListOrSet[T] = list[T] | set[T]")
>>> print(m.body[0].__match_args__)
('name', 'type_params', 'value')

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[JelleZijlstra]

Thanks, good catch. I wonder if something in CI should validate that the __slots__ match existing attribute names.