[C API] Add Py_SETREF() and Py_XSETREF() macros to the limited C API 3.16

[vstinner]

See the capi-workgroup/decisions#110 and the previous issue gh-99574 for the rationale.

The change has been approved by the C API Working Group.

Linked PRs

• gh-158842: Add Py_SETREF() and Py_XSETREF() to the limited C API #158843

[picnixz]

I'm still going to add the type-feature label so forgive me in advance if this blocks your PR :)