Clarify in the documentation that we occasionally add APIs for newer Python...

Clarify in the documentation that we occasionally add APIs for newer Python versions.  Thanks to Russell, Jannis, and Carl for help with the language.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14248 bcc190cf-cafb-0310-a4f2-bffc1f526a37