1. go to https://docs.python.org/2/library/stringio.html (note the "/2/")
2. click on "You should upgrade and read the Python documentation for the current stable release." at the top

I expect to be sent to the Python 3 documentation for StringIO (or at least the index.html page of the Python 3 documentaion) but because StringIO was moved from its own library into `io`, the Python 2 docs are linking to a 404 error page, because they just rewrite "/2/" to "/3/" in the URL.

See attached PR for a list of all these broken pages and where they could redirect to.