Make a key for caching files in the LRU cache.

def _make_key(self):
        """Make a key for caching files in the LRU cache."""
        value = (self._opener,
                 self._args,
                 'a' if self._mode == 'w' else self._mode,
                 tuple(sorted(self._kwargs.items())))
        return _HashedSequence(value)