This reverts commit d52b306f.

Reason for revert: Broke ANGLE CQ with "ImportError: No module named six"
Example: https://ci.chromium.org/p/angle/builders/try/mac-dbg/6758

Original change's description:
> Reland "depot_tools: Move six to depot_tools' .vpython"
> 
> 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/+/1732880
> Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
> Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

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

Change-Id: Id4488249b607441df9f00aa51f4a1f2bc4ad5cfd
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 984182
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1739582Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>