gh-139716: Use the same StackRef flagging scheme for immortals on FT build as on GIL build

[efimov-mikhail]

• Issue: Add tests on basic PyStackRef functions #139716

[efimov-mikhail]

I propose to merge #139717 first and then merge this PR with a proper test change.
IMO, it'd be better to have exactly the same stackref flags on 3 different builds (GIL, FT, STACKREF_DEBUG) to reduce cognitive load.

[Fidget-Spinner]

We'll need to benchmark this, as it may cause a slowdown with the extra check.

[Fidget-Spinner]

I think we should just set immortal objects as having deferred refcount too in _Py_SetImmortal, that way this iwll just be a single check. Though it's been some time since I worked on FT so I don't know the implications of that (I think there should be none?)

[Fidget-Spinner]

We'll need to benchmark this, as it may cause a slowdown with the extra check.

I'll run a benchmark eventually, before we merge.

[colesbury]

This scheme slows things down. See:

• #127024

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[efimov-mikhail]

Ok, I've got it.
What do you think about going in the opposite direction?
#143024