Internal: Add type annotations on Message return

Author: hoh

aleph_message/models/__init__.py

@@ -256,7 +256,8 @@ def check_content(cls, v, values):
         return v
 
 
-def Message(**message_dict: Dict):
+def Message(**message_dict: Dict
+            ) -> Union[PostMessage, AggregateMessage, StoreMessage, ProgramMessage]:
     "Returns the message class corresponding to the type of message."
     for raw_type, message_class in {
         MessageType.post: PostMessage,