Add a pipenv environment for running plugins.py

Pipenv creates a virtual Python 3 environment containing all the
dependencies for running and developing the plugins.py script:

- pygerrit2 (Gerrit REST API client)
- flake8 (Python code analyzer)
- black (Python code formatter)

Update the instructions to use pipenv instead of installing the
dependencies on the host system.

Change-Id: I45c394facdd285a0eca1125a3eeedfd5da29e2d5
3 files changed