depot_tools: Run auth, cipd_bootstrap, git_number tests on python3.

Bug: 1009809
Change-Id: Ia02460a24f4ac048bd7f0072e70856f9ee22a997
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1925314
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>

tests/auth_test.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # 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.

tests/cipd_bootstrap_test.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright (c) 2018 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.

tests/git_number_test.py

-#!/usr/bin/env python
+#!/usr/bin/env vpython3
 # Copyright 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.