Change abc.PrivateChannel to be a proper subclass

This fixes isinstance(thread, discord.abc.PrivateChannel) from
returning True

Author: Rapptz

discord/abc.py

@@ -272,8 +272,7 @@ def mentioned_in(self, message: Message) -> bool:
         raise NotImplementedError
 
 
-@runtime_checkable
-class PrivateChannel(Snowflake, Protocol):
+class PrivateChannel:
     """An ABC that details the common operations on a private Discord channel.
 
     The following implement this ABC:

discord/channel.py

@@ -2381,7 +2381,7 @@ async def create_webhook(self, *, name: str, avatar: Optional[bytes] = None, rea
         return Webhook.from_state(data, state=self._state)
 
 
-class DMChannel(discord.abc.Messageable, Hashable):
+class DMChannel(discord.abc.Messageable, discord.abc.PrivateChannel, Hashable):
     """Represents a Discord direct message channel.
 
     .. container:: operations
@@ -2533,7 +2533,7 @@ def get_partial_message(self, message_id: int, /) -> PartialMessage:
         return PartialMessage(channel=self, id=message_id)
 
 
-class GroupChannel(discord.abc.Messageable, Hashable):
+class GroupChannel(discord.abc.Messageable, discord.abc.PrivateChannel, Hashable):
     """Represents a Discord group channel.
 
     .. container:: operations