> I'd like to add most modifications to 3.4 to simplify backporting.

I would prefer to not touch Python 3.4 backport just to simplify backporting. For example, I don't expect "from __future__ import division" in new Python 3.4 modules, but only on old modules.

If the Python 3.4 evolves, you can use tools like meld to compare your backport and the latest version, and merge manually new changes. Or read Mercurial history of the Python 3.4 module to get patches.

For example, I maintain the faulthandler like that. I manually merge changes when I fix an issue in the CPython version. The version on PyPI is different: it uses SIGALRM instead of a thread, use PyInt_xxx() functions on Python 2, etc.

Do we expect many changes in the statistics modules?