GH-115776: Rename Py_TPFLAGS_INLINE_VALUES as _Py_TPFLAGS_INLINE_VALUES

[markshannon]

The Py_TPFLAGS_INLINE_VALUES should not be used when declaring a static PyTypeObject. It can only be used by the VM when creating a class. It is undocumented, but is exposed in object.h.
This PR adds an underscore prefix to the name to make it clearer that this flag is internal.

• Issue: Inline values array into the object #115776

[Fidget-Spinner]

There are 2 package using this, but otherwise LGTM https://grep.app/search?q=Py_TPFLAGS_INLINE_VALUES

[markshannon]

That's unfortunate. I still think we are better off moving it, as any code using it (except for tools like PySpy) is broken.
I'll deprecate Py_TPFLAGS_INLINE_VALUES instead of removing it, and add a "what's new" entry.

[Fidget-Spinner]

Yeah I realised their usage is wrong. Don't think it's even safe to unset the flag after it's been set.

[encukou]

How do you check whether a type uses this layout (as in JPype)?

The code in JPype is a workaround for earlier breakage. Is there a clean way to do what they need?

[markshannon]

Probably too late to get any code changes in for 3.14, and maybe too late to remove it for 3.15.