[c-c] fix RuntimeError: 'maximum recursion depth exceeded while calling a Python object' we get when creating/upgrading/shelling an instance: hasattr() call __getattribute__, creating an infinite recursion error catched by the interpretor. Avoid this by testing the method is available on the class instead of the instance

doc/book usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc