'Generate the assertion that will be used in the request.'

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)
