Gets key, value pair for oldest item in cache

        :returns: tuple

def oldest(self):
        """
        Gets key, value pair for oldest item in cache

        :returns: tuple
        """
        if len(self._store) == 0:
            return

        kv = min(self._store.items(), key=lambda x: x[1][1])

        return kv[0], kv[1][0]