Add __all__ for modules beginning with 'u', 'x', 'w' and 'z' (#7374)

stdlib/uu.pyi

@@ -1,6 +1,8 @@
 import sys
 from typing import BinaryIO, Union
 
+__all__ = ["Error", "encode", "decode"]
+
 _File = Union[str, BinaryIO]
 
 class Error(Exception): ...