Mypy: Add "dummy import" to avoid cyclic import

[JimChengLin]

If I simply want to import some classes to make proper type annotations, why should I deal with cyclic import issue? Cuz, the code involved is kind of "useless" i.e. deletion would not change anything.

So, I proposal that we can have a special mechanism, like from a import A as A.

Do you see the point? import A as A, which is totally meaningless. Anyone who has a brain would not do it and the interpreter will know this import is just for type annotations.

The change is fairly trivial.

[gvanrossum]

You can do this by using 'if False: import...'

[JimChengLin]

That is even simpler. I am pretty sure that many people do not know it.