Add missing PyObject_GC_Track call, causing *some* itertools.tee objects to

not be tracked by GC. This fixes 254 of test_generators' refleaks on my
machine, but I'm sure something else will make them come back :>

Not adding a separate test for this kind of cycle, since the existing
fib/m235 already test them in more extensive ways than any 'minimal' test
has been able to manage.

Modules/itertoolsmodule.c

@@ -498,6 +498,7 @@ tee_copy(teeobject *to)
 	newto->dataobj = to->dataobj;
 	newto->index = to->index;
 	newto->weakreflist = NULL;
+	PyObject_GC_Track(newto);
 	return (PyObject *)newto;
 }