Introduce an Intersection

This question has already been discussed in #18 long time ago, but now I stumble on this in a practical question: How to annotate something that subclasses two ABC's. Currently, a workaround is to introduce a "mix" class:

from typing import Iterable, Container

class IterableContainer(Iterable[int], Container[int]):
    ...

def f(x: IterableContainer) -> None: ...

class Test(IterableContainer):
    def __iter__(self): ...
    def __contains__(self, item: int) -> bool: ...

f(Test())

but mypy complains about this

error: Argument 1 of "__contains__" incompatible with supertype "Container"

But then I have found this code snippet in #18

def assertIn(item: T, thing: Intersection[Iterable[T], Container[T]]) -> None:
    if item not in thing:
        # Debug output
        for it in thing:
            print(it)

Which is exactly what I want, and it is also much cleaner than introducing an auxiliary "mix" class. Maybe then introducing Intersection is a good idea, @JukkaL is it easy to implement it in mypy?

先阅读 issue #18 中之前的讨论以及本 issue 中的示例，然后查看这里尚未解决的讨论。该 issue 未指明任何文件、测试或入口点；要完成此任务，需要就设计达成一致，并实现支持所演示用例的 Intersection。