You forgot perhaps the most important one:

3. six

`six` was instrumental in repairing the Python schism by giving people a way to incrementally move their 2.7 code to Python 3, and write code that was compatible in both. The six project didn't exist at first and the path to Python 3 was too painful without it. Six solved all that by smoothing over built-in libraries with different casing between versions, incompatible core libraries, the addition of unicode strings, print changing to a function, etc, etc. Perl 5 to Perl 6 (aka Raku) never got that.

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.