gh-106572: Convert PyObject_DelAttr() to a function (#106611)

* Convert PyObject_DelAttr() and PyObject_DelAttrString() macros to
  functions.
* Add PyObject_DelAttr() and PyObject_DelAttrString() functions to
  the stable ABI.
* Replace PyObject_SetAttr(obj, name, NULL) with
  PyObject_DelAttr(obj, name).

Objects/object.c

@@ -942,6 +942,12 @@ PyObject_SetAttrString(PyObject *v, const char *name, PyObject *w)
     return res;
 }
 
+int
+PyObject_DelAttrString(PyObject *v, const char *name)
+{
+    return PyObject_SetAttrString(v, name, NULL);
+}
+
 int
 _PyObject_IsAbstract(PyObject *obj)
 {
@@ -1185,6 +1191,12 @@ PyObject_SetAttr(PyObject *v, PyObject *name, PyObject *value)
     return -1;
 }
 
+int
+PyObject_DelAttr(PyObject *v, PyObject *name)
+{
+    return PyObject_SetAttr(v, name, NULL);
+}
+
 PyObject **
 _PyObject_ComputedDictPointer(PyObject *obj)
 {