I've updated PEP 803 (“abi3t”: Stable ABI for Free-Threaded Builds) based on discussion that started in the previous issue.

Proposed API changes:

• Users set Py_TARGET_ABI3T to build against abi3t, which is like abi3 but for free-threaded builds.

  Implementation-wise, setting Py_TARGET_ABI3T will set:

  • Py_LIMITED_API (to the same version, or minimum if the user defined both) and
  • _Py_OPAQUE_PYOBJECT, which hides some API.

  If both Py_LIMITED_API and Py_GIL_DISABLED are set, CPython headers set Py_TARGET_ABI3T automatically.

• Py_mod_abi becomes mandatory if the new PyModExport_* hook is used

PyMutex/PyCriticalSection is left for a separate discussion. (Without locks, abi3t modules can work correctly but need Py_MOD_GIL_USED in practice.)

Should we recommend this to the SC?

• @encukou
• @erlend-aasland
• @serhiy-storchaka
• @vstinner
• @zooba