[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)".

addressbook
basket
blog
book
calendar
comment
company
email
file
folder
i18ncontent
keyword
link
mailinglist
person
tag
timeseries
vcsfile