Support for pipenv

Pipenv does not properly invoke keyring when locking the file. It can potentially be due to the lock file being generated in a virtual environment that lacks the credential helper.

The workaround here is a bit of a hack:
- Since pipenv supports the addition of environment variables, you can add an environment variable that invokes the gcloud command to print the access token into the pipfile.

Steps:
- export PASSWORD=$(gcloud auth print-access-token)
- add source in Pipfile where username is oauth2accesstoken and password is the environment variable that we had set in the previous step.

At this time, we can't officially support pipenv due to it's architecture.