Get the auth string. If the token is expired and auto refresh enabled,
        a new token will be fetched

        :return: the auth string
        :rtype: str

def auth_string(self):
        """
        Get the auth string. If the token is expired and auto refresh enabled,
        a new token will be fetched

        :return: the auth string
        :rtype: str
        """
        if not self._token:
            self.execute()

        if not self._token.expired:
            return 'Bearer {}'.format(self._token.access_token)

        if self.auto_refresh:
            self.execute()
            return 'Bearer {}'.format(self._token.access_token)

        raise TokenExpired()