[web/views/rdf] Take the second element of XY equivalent for non final relation When given a Yams snippet of the form `<etype> <rtype>` ``xy.xeq()`` will return the XML snippet equivalent to this relation in the form of a tuple `(subject, rtype, object)` so ``item`` will always be a tuple here. In fact, the correct code appears just a few lines above (around line 88) for final relations so I guess this is a copy-paste mistake. Closes #4745929.

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