make repeat_next ft safe

Author: eendebakpt

Modules/itertoolsmodule.c

@@ -3630,10 +3630,13 @@ static PyObject *
 repeat_next(PyObject *op)
 {
     repeatobject *ro = repeatobject_CAST(op);
-    if (ro->cnt == 0)
+    Py_ssize_t cnt = FT_ATOMIC_LOAD_SSIZE_RELAXED(ro->cnt);
+    if (cnt == 0) {
         return NULL;
-    if (ro->cnt > 0)
-        ro->cnt--;
+    }
+    cnt--;
+    assert(cnt >=0);
+    FT_ATOMIC_STORE_SSIZE_RELAXED(ro->cnt, cnt);
     return Py_NewRef(ro->element);
 }