code: async def set_topic(self, channel, topic):
          
          if not self.is_channel(channel):
              raise ValueError('Not a channel: {}'.format(channel))
          elif not self.in_channel(channel):
              raise NotInChannel(channel)
  
          await self.rawmsg('TOPIC', channel, topic)
docs: Set topic on channel.
          Users should only rely on the topic actually being changed when receiving an on_topic_change callback.