Add message attribute to class BaseException.

stdlib/3/builtins.pyi

@@ -728,6 +728,7 @@ Ellipsis = ...  # type: ellipsis
 
 class BaseException:
     args = ...  # type: Any
+    message = ...  # type: str
     def __init__(self, *args: Any) -> None: ...
     def with_traceback(self, tb: Any) -> BaseException: ...