BibDocFile: pickle support fix
- Fixes errors when trying to pickle BibDocFile objects due to the weakref introduced in 4acc073172179038db21ec561e7f4fdf57fceec5. (PR #2549)
Signed-off-by: Esteban J. G. Gabancho <esteban.gabancho@gmail.com>
Reviewed-by: Samuele Kaplun <samuele.kaplun@cern.ch>