code: def servertoken(self,serverURL,referer):
          
          if self._server_token is None or self._server_token_expires_on is None or \
             datetime.datetime.now() >= self._server_token_expires_on or \
             self._server_url != serverURL:
              self._server_url = serverURL
              result = self._generateForServerTokenSecurity(serverURL=serverURL,
                                                            token=self.token,
                                                            tokenUrl=self._token_url,
                                                            referer=referer)
              if 'error' in result:
                  self._valid = False
                  self._message = result
              else:
                  self._valid = True
                  self._message = "Server Token Generated"
          return self._server_token
docs: returns the server token for the server