Upgrade unittest asserts

[hugovk]

failUnlessEqual is set to be removed in Python 3.12 (due October 2023), for example:

• https://docs.python.org/3/whatsnew/3.11.html#pending-removal-in-python-3-12
• https://docs.python.org/3/library/unittest.html#deprecated-aliases

And upgrade other asserts, by using other unittest methods which will give better error messages. For example:

-self.assertTrue(2 in self.lazy)
+self.assertIn(2, self.lazy)