code: def check_username_uniqueness(self, username):
          
          log.info("[+] Checking for Uniqueness of username '{}'".format(username))
          return self._send_xmpp_element(sign_up.CheckUsernameUniquenessRequest(username))
docs: Checks if the given username is available for registration.
          Results are returned in the on_username_uniqueness_received() callback
  
          :param username: The username to check for its existence