Try to 301 old pages when they dissapear

[JulienPalard]

Old documentation pages may dissapear, leading to 404s, like
https://docs.python.org/3/howto/webservers.html.

We should redirect users to the latest version where the page existed, typically for this example, to https://docs.python.org/3.4/howto/webservers.html.

[JulienPalard]

yay someone did it: python/psf-salt#201 ☺