[schema] fix pb with ambiguous relation used within UniqueTogetherConstraint by having CWUniqueTogetherConstraint.relation targeting CWRType instead of CWAttribute/CWRelation. This fixes 3.10.7 migration.

if versions_map['cubicweb'][0] == (3, 7, 4):
    config['http-session-time'] *= 60
    config['cleanup-session-time'] *= 60
    config['cleanup-anonymous-session-time'] *= 60