Use PyFrozenDict_Check() in dict_unhashable_type()

Replace PyObject_IsInstance(op, &PyFrozenDict_Type) with
PyFrozenDict_Check().

Author: vstinner

Objects/dictobject.c

@@ -2407,7 +2407,7 @@ dict_unhashable_type(PyObject *op, PyObject *key)
     }
 
     const char *errmsg;
-    if (PyObject_IsInstance(op, (PyObject*)&PyFrozenDict_Type)) {
+    if (PyFrozenDict_Check(op)) {
         errmsg = "cannot use '%T' as a frozendict key (%S)";
     }
     else {