Indicate py3 compatibility for update_depot_tools_toggle.py

This file is used by a few dependent projects. Highlighting py3
compatiblity eases the migration process for those projects.

This CL additionally adds executation permissions to the file and
includes the script in the docs.

Bug: 1298903
Change-Id: I5838fedcef181ee5e3eb84a1fa16b649250460ea
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/3479854
Auto-Submit: Alexander Schulze <alexschulze@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Reviewed-by: Josip Sokcevic <sokcevic@google.com>
Commit-Queue: Josip Sokcevic <sokcevic@google.com>

README.md

@@ -26,7 +26,8 @@ There are a lot of git utilities included.
 ## Updating
 
 `depot_tools` updates itself automatically when running `gclient` tool. To
-disable auto update, set the environment variable `DEPOT_TOOLS_UPDATE=0`.
+disable auto update, set the environment variable `DEPOT_TOOLS_UPDATE=0` or
+run `./update_depot_tools_toggle.py --disable`.
 
 To update package manually, run `update_depot_tools.bat` on Windows,
 or `./update_depot_tools` on Linux or Mac.

update_depot_tools_toggle.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (c) 2017 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.