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: Bert Vermeulen <bert@biot.com>
Link: https://lore.kernel.org/r/20210121085412.265400-1-bert@biot.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>