fix spurious test-case failure when Python is not installed

Fixes #530.

* tests/core/ltlsynt2.test: Skip when PYTHON is empty.
* NEWS: Mention the fix.

tests/core/ltlsynt2.test

@@ -1,6 +1,6 @@
 #! /bin/sh
 # -*- coding: utf-8 -*-
-# Copyright (C) 2022 Laboratoire de Recherche et Développement de
+# Copyright (C) 2022, 2023 Laboratoire de Recherche et Développement de
 # l'Epita (LRDE).
 #
 # This file is part of Spot, a model checking library.
@@ -36,6 +36,8 @@ ltlsynt --ins=i1,i2 -F formulas.ltl -f 'o1 & F(i1 <-> o2)' -q --csv=out.csv &&\
     exit 2
 test $? -eq 1 || exit 2
 
+test -z "$PYTHON" && exit 77
+
 cat >test.py <<EOF
 import sys
 try: