Nailgun test_update_restricted_networks fails in python2.7

[Bulat Gaifullin]

self = <nailgun.test.integration.test_network_manager.TestNetworkManager testMethod=test_update_restricted_networks>

    def test_update_restricted_networks(self):
        restricted_net = {
            'name': 'restricted_net',
            'restrictions': [
                'settings:additional_components.ironic.value == false'
            ]
        }
        attributes_metadata = """
                editable:
                    additional_components:
                        ironic:
                            value: %r
                            type: "checkbox"
            """
        rel = self.env.create_release()
        netw_meta = deepcopy(rel.networks_metadata)
        netw_meta['neutron']['networks'].append(restricted_net)
        rel.networks_metadata = netw_meta
        cluster = self.env.create_cluster(
            release_id=rel.id,
            api=False
        )
        self.env.create_node(cluster_id=cluster.id)
        self.assertEqual(len(filter(lambda ng: ng.name == 'restricted_net',
                                    cluster.network_groups)), 0)
        with patch.object(logger, 'warning') as mock_warn:
            objects.Cluster.patch_attributes(
                cluster, yaml.load(attributes_metadata % True))
> mock_warn.assert_called_once()

self = <MagicMock name='warning' id='4605409808'>, name = 'assert_called_once'

    def __getattr__(self, name):
        if name in ('_mock_methods', '_mock_unsafe'):
            raise AttributeError(name)
        elif self._mock_methods is not None:
            if name not in self._mock_methods or name in _all_magics:
                raise AttributeError("Mock object has no attribute %r" % name)
        elif _is_magic(name):
            raise AttributeError(name)
        if not self._mock_unsafe:
            if name.startswith(('assert', 'assret')):
> raise AttributeError(name)
E AttributeError: assert_called_once
AttributeError