spdxcheck.py: Use Python 3

Python 2.x has been officially EOL'ed for some time, and in any case
the git module for it is hard to come by.
Signed-off-by: NBert Vermeulen <bert@biot.com>
Link: https://lore.kernel.org/r/20210121085412.265400-1-bert@biot.comSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

scripts/spdxcheck.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 # SPDX-License-Identifier: GPL-2.0
 # Copyright Thomas Gleixner <tglx@linutronix.de>