There was a bug introduced in googleapis/google-auth-library-python#948 where urllib3 was introduced as a dependency without being declared in setup.py here. This results in ModuleNotFoundError: No module named 'urllib3' in environments where the urllib3 package is not installed. Unfortunately, the environment that runs the github presubmits includes the requests package which pulled in urllib3. We should create a separate test environment with only required dependencies.