test_fdt: Move to use Python 3

Update this test to use Python 3 to meet the 2020 deadline.
Signed-off-by: NSimon Glass <sjg@chromium.org>

tools/dtoc/test_fdt.py

-#!/usr/bin/python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0+
 # Copyright (c) 2018 Google, Inc
 # Written by Simon Glass <sjg@chromium.org>