Update type of "throw" method of generators to reflect that it can return a value (#1672)

stdlib/2/typing.pyi

@@ -115,7 +115,7 @@ class Generator(Iterator[_T_co], Generic[_T_co, _T_contra, _V_co]):
     @abstractmethod
     def throw(self, typ: Type[BaseException], val: Optional[BaseException] = None,
               # TODO: tb should be TracebackType but that's defined in types
-              tb: Any = None) -> None: ...
+              tb: Any = None) -> _T_co: ...
 
     @abstractmethod
     def close(self) -> None: ...