[rmdashrfstar]

Major version changes are for API compatibility. If there is a change which makes all other 3.* incompatible, then it should be a major version increase.

[eatonphil]

You're describing something like semver, but Python doesn't do semver.

[dmurray]

And this change would be fine even if Python did.

[gnulinux]

Who told you that? I'm looking at PEP 606, it says nothing like what you claim. https://www.python.org/dev/peps/pep-0606/