#4071 Mixin to prevent setting return_value after initializing certain Mock objects (#4963)

* Addressing #4071 Wrote an ImmutableReturnMixin to prevent developers overriding return_value in certain Mock objects

* Language

* Loosening the assumption that underlying _mock objects need to be Immutable-like simplifies implementation

* Addressing #4071

* Ensure side_effects and return_values are pushed down to the underlying _mock in FreezableMocks. And IDisplay mocks are no longer frozen in _create_get_utility_mock()

* Edit a handful of tests to not override the mock_get_utility return_value

* Brief explainer of FreezableMock.__setattr__

* Incorporating PR feedback and some compatibility

* FreezableMock __getattr__ needs a shortcut in case of return_value or side_effect

* Changing return_value only forbidden if set before freezing

* Remove unnecessary else block

* Expanded doc strings

* Bring a couple new tests in line with patch_get_utility() norms

certbot/plugins/standalone_test.py

@@ -158,10 +158,11 @@ class AuthenticatorTest(unittest.TestCase):
 
     @test_util.patch_get_utility()
     def test_perform_eaddrinuse_retry(self, mock_get_utility):
+        mock_utility = mock_get_utility()
         errno = socket.errno.EADDRINUSE
         error = errors.StandaloneBindError(mock.MagicMock(errno=errno), -1)
         self.auth.servers.run.side_effect = [error] + 2 * [mock.MagicMock()]
-        mock_yesno = mock_get_utility.return_value.yesno
+        mock_yesno = mock_utility.yesno
         mock_yesno.return_value = True
 
         self.test_perform()
@@ -169,7 +170,8 @@ class AuthenticatorTest(unittest.TestCase):
 
     @test_util.patch_get_utility()
     def test_perform_eaddrinuse_no_retry(self, mock_get_utility):
-        mock_yesno = mock_get_utility.return_value.yesno
+        mock_utility = mock_get_utility()
+        mock_yesno = mock_utility.yesno
         mock_yesno.return_value = False
 
         errno = socket.errno.EADDRINUSE