Fix the return type of os.popen() (#1820)

stdlib/2/os/__init__.pyi

@@ -274,7 +274,7 @@ if sys.version_info >= (3, 0):
                      bufsize: int = ...) -> None: ...
         def close(self) -> Any: ...  # may return int
 else:
-    def popen(command: str, *args, **kwargs) -> Optional[IO[Any]]: ...
+    def popen(command: str, *args, **kwargs) -> IO[Any]: ...
     def popen2(cmd: str, *args, **kwargs) -> Tuple[IO[Any], IO[Any]]: ...
     def popen3(cmd: str, *args, **kwargs) -> Tuple[IO[Any], IO[Any], IO[Any]]: ...
     def popen4(cmd: str, *args, **kwargs) -> Tuple[IO[Any], IO[Any]]: ...