[3.13] gh-140471: Fix buffer overflow in AST node initialization with malformed _fields (GH-140506) (#140510)

(cherry picked from commit 95953b692d)

Python/Python-ast.c

@@ -5203,7 +5203,7 @@ ast_type_init(PyObject *self, PyObject *args, PyObject *kw)
                 else {
                     if (PyErr_WarnFormat(
                         PyExc_DeprecationWarning, 1,
-                        "Field '%U' is missing from %.400s._field_types. "
+                        "Field %R is missing from %.400s._field_types. "
                         "This will become an error in Python 3.15.",
                         name, Py_TYPE(self)->tp_name
                     ) < 0) {
@@ -5238,7 +5238,7 @@ ast_type_init(PyObject *self, PyObject *args, PyObject *kw)
                 // simple field (e.g., identifier)
                 if (PyErr_WarnFormat(
                     PyExc_DeprecationWarning, 1,
-                    "%.400s.__init__ missing 1 required positional argument: '%U'. "
+                    "%.400s.__init__ missing 1 required positional argument: %R. "
                     "This will become an error in Python 3.15.",
                     Py_TYPE(self)->tp_name, name
                 ) < 0) {