Add basic yield from type inference. References #647.

test/completion/pep0484_comments.py

@@ -49,7 +49,7 @@ class Employee:
 
 # The typing library is not installable for Python 2.6, therefore ignore the
 # following tests.
-# python > 2.6
+# python >= 2.7
 
 from typing import List
 x = []   # type: List[Employee]