test_fdt: Move to use Python 3

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

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/tools/dtoc/test_fdt.py b/tools/dtoc/test_fdt.py
index 028c8cb..3316757 100755
--- a/tools/dtoc/test_fdt.py
+++ b/tools/dtoc/test_fdt.py
@@ -1,4 +1,4 @@
-#!/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>