Since the issue was dug up, I've created a patch for documentation.

I still think, that it is worth stating the equality is not preserved for sentinels when copied as the message produced by tests is rather confusing:

    class SentinelTestCase(TestCase):
        def test_sentinel(self):
            self.assertEqual(sentinel.foo, copy(sentinel.foo))
            # AssertionError: sentinel.foo != sentinel.foo