Add NoReturn type

[ilevkivskyi]

Fixes #165

Note that here I prohibit List[NoReturn], but maybe we should allow this in order to allow Union[int, NoReturn] and/or Callable[..., NoReturn] (or make exceptions for the latter two).

As discussed in #165, this PR should wait until Python 3.6.1 is released.

[JelleZijlstra]

What would the use of Union[int, NoReturn] be? We could type functions that either return int or throw an exception (e.g., the int constructor) this way, but I'm not sure that would enable better type checking.

Callable[..., NoReturn] makes more logical sense to me, but it doesn't seem like something that's often useful in practice.

[gvanrossum]

I think we've learned in the past that typing.py should be kept simple, stupid. A static checker can reject List[NoReturn].

[ilevkivskyi]

@gvanrossum

I think we've learned in the past that typing.py should be kept simple, stupid.

Yes, right :-) Since this case is not obvious, we can let type checkers decide.

[gvanrossum]

Code LGTM. I wonder though if we should get Mark Shannon's view of the PEP 484 changes first.

[ilevkivskyi]

There is no hurry with this (this could not go into 3.6.1), let us first sort out the PEP, and then I could merge this.

[gvanrossum]

Feel free to merge as soon as 3.6.1 is released. (And typing 3.6.1 has been pushed to PyPI.)