'gmail_oauth2' invocation gives ModuleNotFoundError: No module named 'six'

[rrwoods]

I attempted to run the included gmail_oauth2 script to generate a refresh token for my GCP project so that I could include it in my Django project's environment and use this gmail backend, but I got the error in the title. It seems there is an unlisted dependency.

I worked around the problem by running pip install six.