code: def _generate_assertion(self):
          
          now = int(time.time())
          payload = {
              'aud': self.token_uri,
              'scope': self._scopes,
              'iat': now,
              'exp': now + self.MAX_TOKEN_LIFETIME_SECS,
              'iss': self._service_account_email,
          }
          payload.update(self._kwargs)
          return crypt.make_signed_jwt(self._signer, payload,
                                       key_id=self._private_key_id)
docs: Generate the assertion that will be used in the request.