See https://phabricator.wikimedia.org/T198998.
page._quality can therefore be an int or a str.
This creates problems when filtering of quality level is used.
user@pc:~/python/core {master}$ python scripts/listpages.py -ql:3,4 -prefixindex:"Page:Popular Science Monthly Volume 49.djvu/99" 1 Popular Science Monthly Volume 49.djvu/99 1 page(s) found user@pc:~/python/core {master}$ python scripts/listpages.py -ql:3,4 -prefixindex:"Page:Popular Science Monthly Volume 49.djvu/100" 0 page(s) found