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>
diff --git a/gclient.py b/gclient.py
index a77deca..6c8f998 100755
--- a/gclient.py
+++ b/gclient.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env vpython
 # Copyright (c) 2012 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.
@@ -94,6 +94,7 @@
 import posixpath
 import pprint
 import re
+import six
 import sys
 import time
 
@@ -116,7 +117,6 @@
 import subprocess2
 import setup_color
 
-from third_party import six
 
 
 # TODO(crbug.com/953884): Remove this when python3 migration is done.