[3.12] gh-102304: Fix Py_INCREF() for limited C API 3.9 (#105553)

* gh-102304: Fix Py_INCREF() for limited C API 3.9 (#105550)

When Python is built in debug mode (Py_REF_DEBUG macro), Py_INCREF()
and Py_DECREF() of the limited C API 3.9 (and older) now call
Py_IncRef() and Py_DecRef(), since _Py_IncRef() and _Py_DecRef() were
added to Python 3.10.

(cherry picked from commit 7ba0fd9)

* gh-102304: Remove Py_INCREF() doc change (#105552)

Py_INCREF() was made compatible again with Python 3.9 and older in
the limited API of Python debug mode.

(cherry picked from commit 58e4b69)

Doc/whatsnew/3.12.rst

@@ -1536,15 +1536,6 @@ Build Changes
   :file:`!configure`.
   (Contributed by Christian Heimes in :gh:`89886`.)
 
-* C extensions built with the :ref:`limited C API <limited-c-api>`
-  on :ref:`Python build in debug mode <debug-build>` no longer support Python
-  3.9 and older. In this configuration, :c:func:`Py_INCREF` and
-  :c:func:`Py_DECREF` are now always implemented as opaque function calls,
-  but the called functions were added to Python 3.10. Build C extensions
-  with a release build of Python or with Python 3.12 and older, to keep support
-  for Python 3.9 and older.
-  (Contributed by Victor Stinner in :gh:`102304`.)
-
 
 C API Changes
 =============

Include/object.h

@@ -611,8 +611,14 @@ PyAPI_FUNC(void) _Py_DecRef(PyObject *);
 static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
 {
 #if defined(Py_REF_DEBUG) && defined(Py_LIMITED_API)
-    // Stable ABI for Python built in debug mode
+    // Stable ABI for Python built in debug mode. _Py_IncRef() was added to
+    // Python 3.10.0a7, use Py_IncRef() on older Python versions. Py_IncRef()
+    // accepts NULL whereas _Py_IncRef() doesn't.
+#  if Py_LIMITED_API+0 >= 0x030a00A7
     _Py_IncRef(op);
+#  else
+    Py_IncRef(op);
+#  endif
 #else
     // Non-limited C API and limited C API for Python 3.9 and older access
     // directly PyObject.ob_refcnt.
@@ -642,9 +648,15 @@ static inline Py_ALWAYS_INLINE void Py_INCREF(PyObject *op)
 #endif
 
 #if defined(Py_REF_DEBUG) && defined(Py_LIMITED_API)
-// Stable ABI for Python built in debug mode
+// Stable ABI for Python built in debug mode. _Py_DecRef() was added to Python
+// 3.10.0a7, use Py_DecRef() on older Python versions. Py_DecRef() accepts NULL
+// whereas _Py_IncRef() doesn't.
 static inline void Py_DECREF(PyObject *op) {
+#  if Py_LIMITED_API+0 >= 0x030a00A7
     _Py_DecRef(op);
+#  else
+    Py_DecRef(op);
+#  endif
 }
 #define Py_DECREF(op) Py_DECREF(_PyObject_CAST(op))