add email._policybase.__all__ (#13041)

add email._policybase.__all__

stdlib/email/_policybase.pyi

@@ -5,6 +5,8 @@ from email.message import Message
 from typing import Generic, Protocol, TypeVar, type_check_only
 from typing_extensions import Self
 
+__all__ = ["Policy", "Compat32", "compat32"]
+
 _MessageT = TypeVar("_MessageT", bound=Message, default=Message)
 
 @type_check_only