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

version=3
opts=uversionmangle=s/(rc|a|b|c)/~$1/ \
http://pypi.debian.net/cubicweb/cubicweb-(.+)\.(?:zip|tgz|tbz|txz|(?:tar\.(?:gz|bz2|xz)))