Add MessageType.is_deletable to fix Messageable.purge failing

Fix #10319

discord/enums.py

@@ -279,6 +279,16 @@ class MessageType(Enum):
     poll_result = 46
     emoji_added = 63
 
+    def is_deletable(self) -> bool:
+        return self not in {
+            MessageType.recipient_add,
+            MessageType.recipient_remove,
+            MessageType.call,
+            MessageType.channel_name_change,
+            MessageType.channel_icon_change,
+            MessageType.thread_starter_message,
+        }
+
 
 class SpeakingState(Enum):
     none = 0