work around sns failures due to default region_name='':
> ERROR s3tests_boto3/functional/test_sns.py::test_account_topic_publish - botocore.exceptions.EndpointResolutionError: Invalid region: region was not a valid DNS name.
> ERROR s3tests_boto3/functional/test_sns.py::test_cross_account_topic_publish - botocore.exceptions.EndpointResolutionError: Invalid region: region was not a valid DNS name.