Keep ISO C90 support

Author: vstinner

src/c/_cffi_backend.c

@@ -6100,6 +6100,9 @@ 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().
     */
+#if PY_VERSION_HEX < 0x030D0000
+    PyObject *s;
+#endif
     int first_char;
     assert(objdescr != NULL && objdescr[0] != 0);   /* non-empty */
     first_char = objdescr[0];
@@ -6120,7 +6123,6 @@ static void _my_PyErr_WriteUnraisable(PyObject *t, PyObject *v, PyObject *tb,
                                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);