docs: def _GenerateAssertion(self):
      """Generates the signed assertion that will be used in the request.
  
      Returns:
        string, signed Json Web Token (JWT) assertion.
      """
      now = int(time.time())
      payload = {
          'aud': RpcHelper.TOKEN_ENDPOINT,
          'scope': 'https://www.googleapis.com/auth/identitytoolkit',
          'iat': now,
          'exp': now + RpcHelper.MAX_TOKEN_LIFETIME_SECS,
          'iss': self.service_account_email
      }
      return crypt.make_signed_jwt(
          crypt.Signer.from_string(self.service_account_key),
          payload)