nb_type_new(): adaption for older Python versions, follow tp_traverse recommendations in test suite

Author: wjakob

src/nb_type.cpp

@@ -1176,7 +1176,9 @@ PyObject *nb_type_new(const type_init_data *t) noexcept {
         *s++ = { Py_tp_members, (void*) members };
 
 #if PY_VERSION_HEX < 0x03090000
-    (void) is_generic; // unsupported on Python 3.8
+    // Features that are unsupported in Python 3.8
+    (void) is_generic;
+    type_vectorcall = nullptr;
 #else
     if (is_generic)
         *s++ = { Py_tp_methods, (void*) class_getitem_method };

tests/test_stl.cpp

@@ -70,6 +70,10 @@ int funcwrapper_tp_traverse(PyObject *self, visitproc visit, void *arg) {
     nb::object f = nb::cast(w->f, nb::rv_policy::none);
     Py_VISIT(f.ptr());
 
+    #if PY_VERSION_HEX >= 0x03090000
+        Py_VISIT(Py_TYPE(self));
+    #endif
+
     return 0;
 }