[schema] Make syntax compatible with print_function yams seems to exec() us sometimes (instead of __import__()), and yams.reader uses print_function nowadays.

etc/init.d
etc/cubicweb.d
etc/bash_completion.d
usr/bin
usr/share/doc/cubicweb-ctl
var/log/cubicweb
var/lib/cubicweb/backup
var/lib/cubicweb/instances