[schema] use json to serialize constraints
Require yams 0.43: constraints are serialized to json, which means we
need to recreate the actual checks in the database on upgrade.
Temporary deps in tox.ini to pull respective changes in yams.
usr/lib/python2*/*-packages/cubicweb/web
usr/lib/python2*/*-packages/cubicweb/wsgi
usr/share/cubicweb/cubes/shared/data
usr/share/cubicweb/cubes/shared/wdoc