devmode-test: switch to python3

This is using chromite modules that require python3 now, so switch it.

BUG=chromium:997354
TEST=CQ passes

Change-Id: Ib5a65f2274d9fd6d5c7c069d14e3e55d1742adc0
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/crostestutils/+/2078031
Tested-by: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Dhanya Ganesh <dhanyaganesh@chromium.org>
Reviewed-by: Sean Abraham <seanabraham@chromium.org>
Commit-Queue: Mike Frysinger <vapier@chromium.org>
diff --git a/devmode-test/devinstall_test.py b/devmode-test/devinstall_test.py
index 08c12db..2859551 100755
--- a/devmode-test/devinstall_test.py
+++ b/devmode-test/devinstall_test.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 # -*- coding: utf-8 -*-
 # Copyright (c) 2013 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be