In this CL:
 - Clarify some comments.
 - Remove some unused imports.
 - Make some style more consistent (e.g. quotes, whitespace)

Tools used: pyflakes, flake8 (most warnings ignored)

Change-Id: Ibfb6733c8d844b3c75a7f50b4f3c1d43afabb0ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1773856Reviewed-by: Andrii Shyshkalov <tandrii@google.com>
Commit-Queue: Quinten Yearsley <qyearsley@chromium.org>