[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

#!/bin/sh -e
if [ "$1" = "purge" ] ; then
        update-rc.d cubicweb remove >/dev/null
fi
 
#DEBHELPER#
 
exit 0