Fix error messages for deleting/setting type attributes

Deleting is always an error and doesn't depend on mutability.

Setting is tested after deleting.

Author: da-woods

Objects/typeobject.c

@@ -1125,15 +1125,16 @@ static PyMemberDef type_members[] = {
 static int
 check_set_special_type_attr(PyTypeObject *type, PyObject *value, const char *name)
 {
-    if (_PyType_HasFeature(type, Py_TPFLAGS_IMMUTABLETYPE)) {
+    
+    if (!value) {
         PyErr_Format(PyExc_TypeError,
-                     "cannot set '%s' attribute of immutable type '%s'",
+                     "cannot delete '%s' attribute of type '%s'",
                      name, type->tp_name);
         return 0;
     }
-    if (!value) {
+    if (_PyType_HasFeature(type, Py_TPFLAGS_IMMUTABLETYPE)) {
         PyErr_Format(PyExc_TypeError,
-                     "cannot delete '%s' attribute of immutable type '%s'",
+                     "cannot set '%s' attribute of immutable type '%s'",
                      name, type->tp_name);
         return 0;
     }