patman: Move to use Python 3

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

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

tools/patman/patman.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0+
 #
 # Copyright (c) 2011 The Chromium OS Authors.