[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.
sync_schema_props_perms('cw_source', syncprops=False)
if schema['BigInt'].eid is None:
add_entity_type('BigInt')