I know this because every change I've heard about is reminiscent of a Perl5 change where backwards compatibility was not broken.
The transition to Python 3 was not handled anywhere as well as it could have been.
There is a reason Go2 isn't copying Python3.
(Strangely they seem to be copying the Perl5 update model even though they don't realize it.)
The thing is that because Python has an unhealthy fixation on “There should only be one way to do things” they rejected things that would have made the transition easier. (Or even less necessary.)
I think it is kind-of telling that someone thought it necessary to create Tauthon.
Tauthon is sort-of applying the Perl5 update model to Python 2.7.