[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)".

if repo.system_source.dbdriver == 'postgres':
    sql('ALTER TABLE appears ADD COLUMN weight float')
    sql('UPDATE appears SET weight=1.0 ')