auth.cloud-client.snippets_test: test_implicit failed

[flaky-bot]

Note: #6331 was also for this test, but it was closed more than 10 days ago. So, I didn't mark it flaky.

---

commit: d6ef341
buildURL: Build Status, Sponge
status: failed

Test output

Traceback (most recent call last):
  File "/workspace/auth/cloud-client/snippets_test.py", line 24, in test_implicit
    snippets.implicit()
  File "/workspace/auth/cloud-client/snippets.py", line 23, in implicit
    from google.cloud import storage
  File "/workspace/auth/cloud-client/.nox/py-3-6/lib/python3.6/site-packages/google/cloud/storage/__init__.py", line 35, in 
    from google.cloud.storage.batch import Batch
  File "/workspace/auth/cloud-client/.nox/py-3-6/lib/python3.6/site-packages/google/cloud/storage/batch.py", line 27, in 
    import six
ModuleNotFoundError: No module named 'six'