Broken yield parsing in python 2.7

[kylec1]

Code:

 def wrapped(*args, **kwargs):
      with new_scope(g, r):
            ret = yield f(*args, **kwargs)

results in error: Parse error before "f".

[o11c]

Duplicate of #498 which I am already working on.

[JukkaL]

We are are currently blocked on this, so I'll implement a quick hack the lets mypy process code such as the example above, but doesn't actually do useful type checking.