[test] fix test_printable_value_bytes with current pygments
An empty span was added in
https://bitbucket.org/birkenfeld/pygments-main/commits/164574c13533195a555181a2b9c685fea2470403
usr/lib/python2*/*-packages/cubicweb/entities/
usr/lib/python2*/*-packages/cubicweb/ext/
usr/share/cubicweb/cubes/
usr/lib/python2*/*-packages/cubicweb/*.py