This reverts commit 3a98df0b.

Reason for revert:
Can't roll into skia and build_internal

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>

TBR=iannucci@chromium.org,mmoss@chromium.org,tandrii@chromium.org,ehmaldonado@chromium.org

Change-Id: I51e502e6f7633a6f26898f1ecbcd86ccf9c7c123
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 984182
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1718996Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>