Add support for querying information about group calls.

discord/client.py

@@ -2651,6 +2651,22 @@ class Client:
         """
         return self.connection._get_voice_client(server.id)
 
+    def group_call_in(self, channel):
+        """Returns the :class:`GroupCall` associated with a private channel.
+
+        If no group call is found then ``None`` is returned.
+
+        Parameters
+        -----------
+        channel: :class:`PrivateChannel`
+            The group private channel to query the group call for.
+
+        Returns
+        --------
+        Optional[:class:`GroupCall`]
+            The group call.
+        """
+        return self.connection._calls.get(channel.id)
 
     # Miscellaneous stuff