python-django, python2-django: denial of service

Previously, a session could be created when anonymously accessing the
django.contrib.auth.views.logout view (provided it wasn’t decorated with
django.contrib.auth.decorators.login_required as done in the admin).
This could allow an attacker to easily create many new session records
by sending repeated requests, potentially filling up the session store
or causing other users’ session records to be evicted.
The django.contrib.sessions.middleware.SessionMiddleware has been
modified to no longer create empty session records.