bpo-39573: Use the Py_TYPE() macro (GH-21433)

Replace obj->ob_type with Py_TYPE(obj).
diff --git a/Objects/genericaliasobject.c b/Objects/genericaliasobject.c
index 4d511a2..87bd1ae 100644
--- a/Objects/genericaliasobject.c
+++ b/Objects/genericaliasobject.c
@@ -20,7 +20,7 @@
     Py_XDECREF(alias->origin);
     Py_XDECREF(alias->args);
     Py_XDECREF(alias->parameters);
-    self->ob_type->tp_free(self);
+    Py_TYPE(self)->tp_free(self);
 }
 
 static int