[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.

Pygments
#fyzz XXX pip install fails
cubicweb-card
cubicweb-file
cubicweb-localperms
cubicweb-tag