legacy: fix six import in bibsort
- Adds missing import of six module, since its usage has been added in 631679635d932a8f16179ca562847cc42a76af9d. (fixes #2929) (closes #2978)
Signed-off-by: Jacopo Notarstefano <jacopo.notarstefano@cern.ch>
Reviewed-by: Jiri Kuncar <jiri.kuncar@cern.ch>