[schema] store default attribute values in a Bytes field, allowing python objects as default values Notes: * Binary objects grow two methods * we depend on a newer yams version * the code is quite simplified as a result of the whole change * a very old and nasty bug where bool(default values) evaluating to False not being properly migrated is killed Closes #2414591.

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