Switch gsutil.py to py3

R=ehmaldonado@chromium.org

Bug: 1199969
Change-Id: If05401f3f6c5c8e302220a52c4c237be0f721da3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2836346Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

gsutil.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright 2014 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.