[web/debug] Fix check for open sessions itervalues returns an iterator, which never evaluates to False. Regression from 0bb18407c053 "[toward py3k] rewrite dict.keys() and dict.values() (part of #2711624)".

missing-dep-for-interpreter make => make | build-essential | dpkg-dev (usr/share/pyshared/cubicweb/skeleton/debian/rules.tmpl)