Add MessageType.is_deletable to fix Messageable.purge failing

Fix #10319

Author: Rapptz

discord/abc.py

@@ -195,6 +195,9 @@ async def _purge_helper(
             count = 0
             await asyncio.sleep(1)
 
+        if not message.type.is_deletable():
+            continue
+
         if not check(message):
             continue
 

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

docs/api.rst

@@ -1732,6 +1732,15 @@ of :class:`enum.Enum`.
 
           Checks if two messages are not equal.
 
+    .. method:: is_deletable()
+
+        Checks if the message type is deletable, as some system messages cannot be deleted.
+
+        .. versionadded:: 2.7
+
+        :return: A boolean denoting if the message type is deletable.
+        :rtype: :class:`bool`
+
     .. attribute:: default
 
         The default message type. This is the same as regular messages.