[migration/3.18] recover from bad CWSource.in_synchronization default value The addition of this attribute in 3.13.8 had default=False, which made no sense, and was fixed in 3.13.9. However no migration was done so apps that ever saw the 3.13.8 code could still have '' as defaultval for this attribute.

raise NotImplementedError("Cannot migrate such an old version. Use intermediate Cubiweb version (try 3.16.x)")