Six was one component but not the only one. Python 2.7 also backported a number of early Python 3 features, Python 2 features were reintroduced in basically every P3 version until at least 3.5 (although after 3.3 they were pretty minor), and a lot of extensive migration guides were written (my main bible was eevee's).

In my experience, six was a relatively minor part, and you could get by with your own little compat file for just the stuff you needed, even on relatively big projects. I even found it beneficial to do so because instead of just slapping six.moves everywhere you'd have to re-evaluate some of the old decisions (e.g. at $dayjob at the time we were using all of urllib, urllib2, and requests for HTTP calls, not using six provided strong motivation to just move everything to requests). This also made for a lot less churn when removing Python 2 compatibility.

> Python 2 features were reintroduced in basically every P3 version until at least 3.5

If they had just done this from the beginning there wouldn't even have been such upgrade drama in the first place... like, as an obvious example, removing u'' syntax for unicode strings immediately at 3.0 was just idiotic: if it weren't for some dumb decisions like that one there would have been almost no upgrade discontinuity at all (a la Ruby 2's Unicode reboot, which concerned a lot of people but was a nothing-burger next to the insanity of Python 3).