Fix traceback when 'pack' appends '.rpz' on PY3
Fix traceback when 'pack' appends '.rpz' on PY3

JosuaKrause/reprozip,JosuaKrause/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip,JosuaKrause/reprozip,JosuaKrause/reprozip,ViDA-NYU/reprozip,ViDA-NYU/reprozip