* Backported itertools.chain.from_iterable for cms.sitemaps.cms_sitemap
* Replaced new-style property usage in cms.models.Placeholder with old style
* Added a missing future import in cms.middleware.multilingual
* Refactored the docs index page to have a separate section for
installation and upgrades at the top.
* Added 2.3.3 release notes that include a deprecation warning for
Python 2.5 in the next release.