Progress on no-GIL CPython

Precisely my first thought. Python4 should be defined as python-nogil. The Python language is the same for both versions, and future language additions would stay in step i.e. python3.15 and python4.15 would always run the same pure Python code without issues.

BUT using a different abi, so python3 libraries won't necessarily be available under python4 to start with (or sometimes, never).

A long, LONG way down the road, python3 might be retired, because there's nothing significant left that python4 can't do.