I left a review on rietveld.

FWIW we discussed this already somewhere, and IIRC the two main issues were:
1) mapping new/renamed/delete files between versions;
2) making this work while building the docs locally;

Redirecting to 'http://docs.python.org/{version}' might be an acceptable solution for 1) if we don't want to keep lists of module names.
Regarding 2) it depends on the local directory layout, so it would need to be configurable.

These are just further enhancements though, I think it would be good to have something similar even if it doesn't cover all the possible cases.