[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.
# Run all scenarii found in windmill directoryfromcubicweb.devtools.cwwindmillimport(CubicWebWindmillUseCase,unittest_main)if__name__=='__main__':unittest_main()