[Typing-sig] Re: Add Expand special form to allow typing **kwargs with a TypedDict

So far I only went through PEP 646 briefly and I am not really convinced if reusing Unpack would be a good thing or not. Will try to give it more thought but for now having a seprate special form like Expand or UnpackKwargs makes more sense to me. On the other hand, from user's perspective being able to reuse Unpack would be more comfortable.