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).

Python/bltinmodule.c

@@ -1567,8 +1567,9 @@ static PyObject *
 builtin_delattr_impl(PyObject *module, PyObject *obj, PyObject *name)
 /*[clinic end generated code: output=85134bc58dff79fa input=164865623abe7216]*/
 {
-    if (PyObject_SetAttr(obj, name, (PyObject *)NULL) != 0)
+    if (PyObject_DelAttr(obj, name) < 0) {
         return NULL;
+    }
     Py_RETURN_NONE;
 }