Move subcommand_test to py3

Change-Id: Iabc6ceb75da8a71f089c8fd89c94890f7b3f428e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2106513
Commit-Queue: Josip Sokcevic <sokcevic@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

tests/subcommand_test.py

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