[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)".
usr/lib/PY_VERSION/*-packages/cubicweb/server/
usr/lib/PY_VERSION/*-packages/cubicweb/hooks/
usr/lib/PY_VERSION/*-packages/cubicweb/sobjects/
usr/lib/PY_VERSION/*-packages/cubicweb/schemas/
usr/share/cubicweb/migration/