Change abc.PrivateChannel to be a proper subclass

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

discord/abc.py

@@ -369,8 +369,7 @@ class User(Snowflake, Protocol):
         raise NotImplementedError
 
 
-@runtime_checkable
+class PrivateChannel:
-class PrivateChannel(Snowflake, Protocol):
     """An ABC that details the common operations on a private Discord channel.
 
     The following implement this ABC:
@@ -388,6 +387,7 @@ class PrivateChannel(Snowflake, Protocol):
 
     __slots__ = ()
 
+    id: int
     me: ClientUser
 
     def _add_call(self, **kwargs):

discord/channel.py

@@ -2301,7 +2301,7 @@ class ForumChannel(discord.abc.GuildChannel, Hashable):
         return Webhook.from_state(data, state=self._state)
 
 
-class DMChannel(discord.abc.Messageable, discord.abc.Connectable, Hashable):
+class DMChannel(discord.abc.Messageable, discord.abc.Connectable, discord.abc.PrivateChannel, Hashable):
     """Represents a Discord direct message channel.
 
     .. container:: operations
@@ -2640,7 +2640,7 @@ class DMChannel(discord.abc.Messageable, discord.abc.Connectable, Hashable):
         await self._state.http.decline_message_request(self.id)
 
 
-class GroupChannel(discord.abc.Messageable, discord.abc.Connectable, Hashable):
+class GroupChannel(discord.abc.Messageable, discord.abc.Connectable, discord.abc.PrivateChannel, Hashable):
     """Represents a Discord group channel.
 
     .. container:: operations