bpo-44263: Fix _decimal and _testcapi GC protocol (GH-26464) (GH-26465)

* _testcapi.heapgctype: implement a traverse function since the type
  is defined with Py_TPFLAGS_HAVE_GC.
* _decimal: PyDecSignalDictMixin_Type is no longer defined with
  Py_TPFLAGS_HAVE_GC since it has no traverse function.
(cherry picked from commit 142e5c5445c019542246d93fe2f9e195d3131686)

Co-authored-by: Victor Stinner <vstinner@python.org>

Co-authored-by: Victor Stinner <vstinner@python.org>
diff --git a/Modules/_testcapimodule.c b/Modules/_testcapimodule.c
index d926ad8..c759dcc 100644
--- a/Modules/_testcapimodule.c
+++ b/Modules/_testcapimodule.c
@@ -6595,6 +6595,13 @@ heapctype_init(PyObject *self, PyObject *args, PyObject *kwargs)
     return 0;
 }
 
+static int
+heapgcctype_traverse(HeapCTypeObject *self, visitproc visit, void *arg)
+{
+    Py_VISIT(Py_TYPE(self));
+    return 0;
+}
+
 static void
 heapgcctype_dealloc(HeapCTypeObject *self)
 {
@@ -6608,6 +6615,7 @@ static PyType_Slot HeapGcCType_slots[] = {
     {Py_tp_init, heapctype_init},
     {Py_tp_members, heapctype_members},
     {Py_tp_dealloc, heapgcctype_dealloc},
+    {Py_tp_traverse, heapgcctype_traverse},
     {Py_tp_doc, (char*)heapgctype__doc__},
     {0, 0},
 };