[3.13] gh-133122: Fix for test_type_lookup_mro_reference. (gh-133127)

Change the unit test case to use `getattr()` so that we avoid the
bytecode specializer optimizing the access.  The specializer will call
the `__eq__` method before the unit test expects, causing it to fail.

In the 3.14 branch (gh-128164) the test is changed in a different way
to avoid the same issue.