Implement PEP 526 in typed_ast (#548)

See python/typed_ast#16.

Author: ilevkivskyi

third_party/3/typed_ast/ast35.pyi

@@ -88,8 +88,9 @@ class Delete(stmt):
 
 class Assign(stmt):
     targets = ...  # type: typing.List[expr]
-    value = ...  # type: expr
+    value = ...  # type: Optional[expr]
     type_comment = ...  # type: Optional[str]
+    annotation = ...  # type: Optional[expr]
 
 class AugAssign(stmt):
     target = ...  # type: expr