Fix detail::obj_class_name() to work correctly for meta classes. (#4436)

* Fix `detail::obj_class_name()` to work correctly for meta classes.

* Adjust expected name for PyPy

tests/test_class.cpp

@@ -55,6 +55,8 @@ void bind_empty0(py::module_ &m) {
 } // namespace test_class
 
 TEST_SUBMODULE(class_, m) {
+    m.def("obj_class_name", [](py::handle obj) { return py::detail::obj_class_name(obj.ptr()); });
+
     // test_instance
     struct NoConstructor {
         NoConstructor() = default;