add-project.py:
  Reformatted with `black`.

.gitlab-ci.yml:
  Add `lint` job in the `verify` stage which validates Python
  sources using `flake8` and `black`.

vendor/*:
  Added wheels for black, flake8, and dependencies.

Change-Id: I30c213b6a21ecc828d726b4dccfaaeb8ea8f0749