binman: Move to use Python 3

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

Unfortunately this introduces a test failure due to a problem in pylibfdt
on Python 3. I will investigate.

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

tools/binman/binman.py

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