Testsuite fails on Python 3.12

[glaubitz]

On Python 3.12, the testsuite fails with the following tests:

[   31s] =========================== short test summary info ============================
[   31s] FAILED tests/unit/common/monitored_resource_util/test_monitored_resource.py::TestMonitoredResource::test_aws_monitored_resource
[   31s] FAILED tests/unit/common/monitored_resource_util/test_monitored_resource.py::TestMonitoredResource::test_gcp_gce_monitored_resource
[   31s] FAILED tests/unit/common/monitored_resource_util/test_monitored_resource.py::TestMonitoredResource::test_gcp_k8s_monitored_resource
[   31s] FAILED tests/unit/stats/test_aggregation_data.py::TestDistributionAggregationData::test_exemplar_int_attachment_key
[   31s] FAILED tests/unit/stats/test_aggregation_data.py::TestDistributionAggregationData::test_exemplar_int_attachment_value
[   31s] FAILED tests/unit/stats/test_aggregation_data.py::TestDistributionAggregationData::test_exemplar_null_attachment_key
[   31s] FAILED tests/unit/stats/test_aggregation_data.py::TestDistributionAggregationData::test_exemplar_null_attachment_value
[   31s] FAILED tests/unit/stats/test_aggregation_data.py::TestDistributionAggregationData::test_exemplar_null_attachments
[   31s] FAILED tests/unit/stats/test_measurement_map.py::TestMeasurementMap::test_put_attachment_int_key
[   31s] FAILED tests/unit/stats/test_measurement_map.py::TestMeasurementMap::test_put_attachment_int_value
[   31s] FAILED tests/unit/stats/test_measurement_map.py::TestMeasurementMap::test_put_attachment_none_key
[   31s] FAILED tests/unit/stats/test_measurement_map.py::TestMeasurementMap::test_put_attachment_none_value
[   31s] ========== 12 failed, 535 passed, 2 deselected, 94 warnings in 4.26s ===========

One of the test failures looks like this:

[   24s] ______________ TestMonitoredResource.test_aws_monitored_resource _______________
[   24s] 
[   24s] self = <test_monitored_resource.TestMonitoredResource testMethod=test_aws_monitored_resource>
[   24s] aws_md_mock = <MagicMock name='AwsIdentityDocumentUtils' id='140226359062464'>
[   24s] 
[   24s]     @mock.patch('opencensus.common.monitored_resource.monitored_resource'
[   24s]                 '.aws_identity_doc_utils.AwsIdentityDocumentUtils')
[   24s]     def test_aws_monitored_resource(self, aws_md_mock):
[   24s]     
[   24s]         mocked_labels = {
[   24s]             'instance_id': 'i-1234567890abcdef0',
[   24s]             'aws_account': '123456789012',
[   24s]             'region': 'us-west-2'
[   24s]         }
[   24s]     
[   24s]         aws_md_mock.return_value = mock.Mock()
[   24s]         aws_md_mock.return_value.get_aws_metadata.return_value = mocked_labels
[   24s]     
[   24s]         with mock_aws_env():
[   24s]             resource = monitored_resource.get_instance()
[   24s]         self.assertEqual(resource.get_type(), 'aws_ec2_instance')
[   24s]         self.assertEqual(resource.get_labels(), mocked_labels)
[   24s]     
[   24s]         with mock_oc_env():
[   24s]             with mock_aws_env():
[   24s]                 resource = monitored_resource.get_instance()
[   24s]         self.assertEqual(resource.get_type(), 'mock_resource_type')
[   24s] >       self.assertDictContainsSubset(
[   24s]             {'mock_label_key': 'mock_label_value'}, resource.get_labels())
[   24s] E       AttributeError: 'TestMonitoredResource' object has no attribute 'assertDictContainsSubset'
[   24s] 
[   24s] tests/unit/common/monitored_resource_util/test_monitored_resource.py:169: AttributeError