Reland "depot_tools: Move six to depot_tools' .vpython"

This is a reland of 3a98df0bdd49d64cb15a5dd0fbc6b43223856e7d

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>
diff --git a/git_cl.py b/git_cl.py
index a52b0ae..54fb2f3 100755
--- a/git_cl.py
+++ b/git_cl.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env vpython
 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.