Nick Coghlan added the comment:
Regarding 2v3, the layout differences aren't a problem, since the canonical URLs are separate (/2/* vs /3/*). That's one of the benefits I actually hope for with this change - due to PEP 430, deep links still go to the Python 2 documentation by default, and once this change is made in the Python 2.7 branch it should teach search engines that those should start being presented in results as "/2/*" qualified links.
We also don't tend to make wholesale changes to the URL layouts in the docs in X.Y releases, so I think the assumption of "the relative path of this page won't change" is fine.
Georg, Berker - any further thoughts before we make this change to 3.4+ and the 2.7 docs?