Explicitly close any associated file object (if necessary).

def close(self, needs_lock=True):
        """Explicitly close any associated file object (if necessary)."""
        # TODO: remove needs_lock if/when we have a reentrant lock in
        # dask.distributed: https://github.com/dask/dask/issues/3832
        with self._optional_lock(needs_lock):
            default = None
            file = self._cache.pop(self._key, default)
            if file is not None:
                file.close()