use <cassert> and not <assert.h> in C++

* spot/mc/unionfind.cc, spot/twacube/cube.cc,
spot/twacube_algos/convert.cc: Includes code.
* tests/sanity/style.test: Add a test for this.

spot/mc/unionfind.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2015, 2016, 2018 Laboratoire de Recherche et
+// Copyright (C) 2015, 2016, 2018, 2021 Laboratoire de Recherche et
 // Developpement de l'Epita
 //
 // This file is part of Spot, a model checking library.
@@ -20,7 +20,7 @@
 
 #include "config.h"
 #include <spot/mc/unionfind.hh>
-#include <assert.h>
+#include <cassert>
 
 namespace spot
 {

spot/twacube/cube.cc

@@ -1,5 +1,5 @@
 // -*- coding: utf-8 -*-
-// Copyright (C) 2015, 2016, 2018, 2020 Laboratoire de Recherche et
+// Copyright (C) 2015, 2016, 2018, 2020, 2021 Laboratoire de Recherche et
 // Developpement de l'Epita (LRDE).
 //
 // This file is part of Spot, a model checking library.
@@ -21,7 +21,7 @@
 #include <sstream>
 #include <iostream>
 #include <spot/twacube/cube.hh>
-#include <assert.h>
+#include <cassert>
 
 namespace spot
 {

spot/twacube_algos/convert.cc

@@ -20,7 +20,7 @@
 #include "config.h"
 #include <spot/twacube_algos/convert.hh>
 #include <spot/twaalgos/contains.hh>
-#include <assert.h>
+#include <cassert>
 
 namespace spot
 {