==================
- Add ``persistent.Persistent`` and ``zope.container.contained.Contained`` as
bases for ``zope.pluggableauth.plugins.session.SessionCredentialsPlugin``,
so instances of ``zope.app.authentication.session.SessionCredentialsPlugin``
won't be changed.
(https://mail.zope.org/pipermail/zope-dev/2010-July/040898.html)