Properly fix Python 3.13 stable ABI linking

This fixes vim#15460 properly. There was an attempt to fix it in v9.1.0668,
but it did it by manually copy and pasting definitions from Python 3.13
headers, which is problematic as it makes Vim dependent on low-level
implementation details which are subject to change. That change also
pulls in dependencies to private APIs (`_Py_IncRef`) which is a very bad
idea as the next version of Python could very well remove that.

The core issue was simply that `Py_INCREF` (similar functions) is not
part of the stable API. We are supposed to be using `Py_IncRef` instead
which performs null-check (similar to `Py_XINCREF`) and is available as
a function. We simply need to call it.

src/if_python3.c

@@ -219,16 +219,9 @@ static HINSTANCE hinstPy3 = 0; // Instance of python.dll
 # define PyObject_GetItem py3_PyObject_GetItem
 # define PyObject_IsTrue py3_PyObject_IsTrue
 # define PyModule_GetDict py3_PyModule_GetDict
-# if defined(USE_LIMITED_API) \
-    && (Py_LIMITED_API+0 >= 0x030c0000 || defined(Py_REF_DEBUG))
-#  undef Py_INCREF
-#  if Py_LIMITED_API+0 >= 0x030a00A7
-#   define _Py_IncRef py3__Py_IncRef
-#   define Py_INCREF _Py_IncRef
-#  else
-#   define Py_IncRef py3_Py_IncRef
-#   define Py_INCREF Py_IncRef
-#  endif
+# ifdef USE_LIMITED_API
+#  define Py_IncRef py3_Py_IncRef
+#  define Py_DecRef py3_Py_DecRef
 # endif
 # ifdef USE_LIMITED_API
 #  define Py_CompileString py3_Py_CompileString
@@ -402,14 +395,9 @@ static void (*py3_Py_Finalize)(void);
 static void (*py3_PyErr_SetString)(PyObject *, const char *);
 static void (*py3_PyErr_SetObject)(PyObject *, PyObject *);
 static int (*py3_PyErr_ExceptionMatches)(PyObject *);
-# if defined(USE_LIMITED_API) \
-    && (Py_LIMITED_API+0 >= 0x030c0000 || defined(Py_REF_DEBUG))
-#  if Py_LIMITED_API+0 >= 0x030a00A7
-#   define _Py_IncRef py3__Py_IncRef
-static void (*py3__Py_IncRef)(PyObject *);
-#  else
+# ifdef USE_LIMITED_API
 static void (*py3_Py_IncRef)(PyObject *);
-#  endif
+static void (*py3_Py_DecRef)(PyObject *);
 # endif
 # ifdef USE_LIMITED_API
 static PyObject* (*py3_Py_CompileString)(const char *, const char *, int);
@@ -618,13 +606,9 @@ static struct
     {"PyErr_SetString", (PYTHON_PROC*)&py3_PyErr_SetString},
     {"PyErr_SetObject", (PYTHON_PROC*)&py3_PyErr_SetObject},
     {"PyErr_ExceptionMatches", (PYTHON_PROC*)&py3_PyErr_ExceptionMatches},
-# if defined(USE_LIMITED_API) \
-    && (Py_LIMITED_API+0 >= 0x030c0000 || defined(Py_REF_DEBUG))
-#  if Py_LIMITED_API+0 >= 0x030a00A7
-    {"_Py_IncRef", (PYTHON_PROC*)&py3__Py_IncRef},
-#  else
+# ifdef USE_LIMITED_API
     {"Py_IncRef", (PYTHON_PROC*)&py3_Py_IncRef},
-#  endif
+    {"Py_DecRef", (PYTHON_PROC*)&py3_Py_DecRef},
 # endif
 # ifdef USE_LIMITED_API
     {"Py_CompileString", (PYTHON_PROC*)&py3_Py_CompileString},
@@ -802,18 +786,18 @@ py3__Py_XDECREF(PyObject *op)
 #  define Py_XDECREF(op) py3__Py_XDECREF(_PyObject_CAST(op))
 # endif
 
-# if defined(USE_LIMITED_API) \
-    && (Py_LIMITED_API+0 >= 0x030c0000 || defined(Py_REF_DEBUG))
-    static inline void
-py3__Py_XINCREF(PyObject *op)
-{
-    if (op != NULL)
-    {
-	Py_INCREF(op);
-    }
-}
-#  undef Py_XINCREF
-#  define Py_XINCREF(op) py3__Py_XINCREF(_PyObject_CAST(op))
+# ifdef USE_LIMITED_API
+// Use stable versions of inc/dec ref. Note that these always null-check and
+// therefore there's no difference between XINCREF and INCREF.
+#   undef Py_XINCREF
+#   define Py_XINCREF(obj) Py_IncRef((PyObject *)obj)
+#   undef Py_INCREF
+#   define Py_INCREF(obj) Py_IncRef((PyObject *)obj)
+
+#   undef Py_DECREF
+#   define Py_DECREF(obj) Py_DecRef((PyObject *)obj)
+#   undef Py_XDECREF
+#   define Py_XDECREF(obj) Py_DecRef((PyObject *)obj)
 # endif
 
 # if PY_VERSION_HEX >= 0x030900b0