This would totally make sense. A major change in architecture with some libraries breaking. While many people are still store from the 2->3 transition I would not expect this change to be that severe Most code should work for python 3 and 4 without changes.

It could be done in incremental steps:

1. python3.x nogil as a compiler flag

2. python3.y nogil as a runtime flag

3. python4.0 nogil as the default (if we’re really ready for it)

Please no. I will quit python if they do another breaking major version change.

It doesn't have to be breaking for a 4.0 label.

Python doesn't follow semver anyway (and it's fine IMO).

Semantic Versioning is basically dead on most open source projects these days. I don't think most even used it right back in the day.