Use PyErr_FormatUnraisable() on Python 3.13

Use the new public PyErr_FormatUnraisable() on Python 3.13.

The private _PyErr_WriteUnraisableMsg() function was removed in
Python 3.13:
python/cpython#111643

src/c/_cffi_backend.c

@@ -6100,7 +6100,6 @@ static void _my_PyErr_WriteUnraisable(PyObject *t, PyObject *v, PyObject *tb,
        Luckily it's also Python 3.8 that adds new functionality that
        people might want: the new sys.unraisablehook().
     */
-    PyObject *s;
     int first_char;
     assert(objdescr != NULL && objdescr[0] != 0);   /* non-empty */
     first_char = objdescr[0];
@@ -6109,6 +6108,18 @@ static void _my_PyErr_WriteUnraisable(PyObject *t, PyObject *v, PyObject *tb,
     if (extra_error_line == NULL)
         extra_error_line = "";
 
+#if PY_VERSION_HEX >= 0x030D0000
+    if (obj != NULL) {
+        PyErr_FormatUnraisable("Exception ignored %c%s%R%s",
+                               first_char, objdescr + 1, obj,
+                               extra_error_line);
+    }
+    else {
+        PyErr_FormatUnraisable("Exception ignored %c%s%s",
+                               first_char, objdescr + 1, extra_error_line);
+    }
+#else
+    PyObject *s;
     if (obj != NULL)
         s = PyUnicode_FromFormat("%c%s%R%s",
             first_char, objdescr + 1, obj, extra_error_line);
@@ -6124,6 +6135,7 @@ static void _my_PyErr_WriteUnraisable(PyObject *t, PyObject *v, PyObject *tb,
     else
         PyErr_WriteUnraisable(obj);   /* best effort */
     PyErr_Clear();
+#endif
 
 #else