Restrict types for stubs in itertools

[mvcisback]

Can use Union type to model Maybes and Products.

[mvcisback]

Good Catch

This is the kind of situation dependent types where would be particularly useful.
I suspect without them I suspect its impossible to accurately represent product.

It might be better to implement instead separate product2, product3, product4, ... and leave itertool's product with a more general type signature.

For now I'm going to update the PR with Iterator[Sequence[T]].

[JukkaL]

Thanks, looks good now! We can iterate on the signatures in the future if the current ones seem too restrictive in practice.