test_dtoc: Move to use Python 3

Update this test to use Python 3 to meet the 2020 deadline.

Also make it executable while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>

tools/dtoc/dtoc.py

@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0+
 #
 # Copyright (C) 2016 Google, Inc

tools/dtoc/test_dtoc.py

@@ -1,3 +1,4 @@
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0+
 # Copyright (c) 2012 The Chromium OS Authors.
 #