class OldIterable(Sized, Protocol[T]):
    def __getitem__(self, item: int) -> T: ...

CompatIterable = Union[Iterable[T], OldIterable[T]]

class A:
    def __iter__(self) -> Iterator[str]: ...
class B:
    def __len__(self) -> int: ...
    def __getitem__(self, item: int) -> str: ...

def iterate(it: CompatIterable[str]) -> None:
    ...

iterate(A())  # OK
iterate(B())  # OK