cubicweb #3007573 drop yams inheritance support [open]

IMO, we don't want this. As we support behaviour inheritance (by using python heritage of entity classes), and we have with RQL a "duck typed" language, we don't need (want) type inheritance.

The only thing that should be kept is the different behaviour when you do yams class inheritance with or without __specializes_schema__: the former only take attributes/relations defined in the scope of the parent class, the later will copy all the attributes and relations. Beside this, we don't need any additional semantic difference.

Benefits include code simplification and removal of the costly is_instance_of relation.

done in<not specified>
closed by<not specified>