I don't know how official it is, but I've used this mirror when I wanted to make use of BitBucket side-by-side comparisons:
https://bitbucket.org/mirror/cpython/
It seems fairly up-to-date. Much of my early work on the venv code was done using a clone from this mirror, and I pulled and merged from it lots of times without any issues.
Atlassian setup https://bitbucket.org/python_mirrors to mirror the entire hg.python.org setup.
http://blog.python.org/2012/06/mercurial-mirrors-provided-by-atlassian.html _______________________________________________
The devguide (http://docs.python.org/devguide/committing.html) says: Bitbucket <http://www.bitbucket.org> also maintain an up to date clone<https://bitbucket.org/mirror/cpython/overview>of the main cpython repository that can be used as the basis for a new clone or patch queue. [the link goes to https://bitbucket.org/mirror/cpython/overview] Eli