This is a reland of 3a98df0b

Original change's description:
> depot_tools: Move six to depot_tools' .vpython
> 
> Bug: 984182
> Change-Id: I34a242c8607624beb0f06a27c563e413dec99790
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1717492
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
> Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Reviewed-by: Michael Moss <mmoss@chromium.org>

Bug: 984182
Change-Id: Ibf7034201e65a18f833cf1e7432f81a903a99aae
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1732880Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>