Stable ABI/Limited API for free-threaded builds

[encukou (Petr Viktorin)]

@steve.dower

Even if we did this, we can’t change the layout of PyObject until whenever we decide that limited API modules built with 3.14 are no longer supported. There isn’t an agreed upon definition of this deprecation timeline (that I’m aware of), so it seems likely that we’ll need a hard break migration at some point before it expires anyway. May as well be planning for that (and supporting it in parallel as much as we can) rather than trying to patch over design decisions from the past.

That hard break is called “Phase III”. No timeline, but it seems it could be pretty soon.

I agree about making the transition as painless as possible. That’s why I propose both API compatibility (no source changes → separate abi3 & abi3t builds) and ABI compatibility (PyModExport, opaque PyObject → abi3.abi3t builds).
We want extensions to switch to the latter (so that we can change PyObject in the future), but I think we need a window where we support both – both for extension authors to switch, and for CPython to react to feedback.