Continuous test failing because of missing dependency six

[dandhlee]

Most continuous tests are failing with the following error message:

Traceback:
/usr/local/lib/python3.8/importlib/__init__.py:127: in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
tests/unit/test_retry.py:17: in <module>
    from google.cloud.storage import _helpers
google/cloud/storage/__init__.py:35: in <module>
    from google.cloud.storage.batch import Batch
google/cloud/storage/batch.py:27: in <module>
    import six
E   ModuleNotFoundError: No module named 'six'