Does this have to be implemented on the doc build of EOL pythons versions (like 2.6), or can it be a script which is ran once on these old docs ? 


One of the issues I had trying to implement that on other projects was that you don't know in advance what the future pages will be or if it will be gone. So you can go this route[1], in which case it's a 10 line fix that I'm happy to contribute also here. 

Typically `https://docs.python.org/2/c-api/string.html` has no cannonical on 3 whouch you can't know while building with sphinx. 

Or I though about injecting an html comment with a html comment that you search and replace once you publish the "new" version and only if the target page exists on the new version. 


So : 

- is that ok to have non existing cannonical ? 
- is that ok if adding <rel>s on docs is (or requires) a post-sphinx script ? 

1: https://github.com/xonsh/xonsh/pull/1914/files

Thanks