scripts: add Python 3 compatibility to spdxcheck.py

"dict.has_key(key)" on dictionaries has been replaced with "key in
dict".  Additionally, when run under Python 3 some files don't decode
with the default encoding (tested with UTF-8).  To handle that, don't
open the file in text mode and decode text line-by-line, ignoring
encoding errors.

This remains compatible with Python 2 and should have no functional
change.

Link: http://lkml.kernel.org/r/20180717190635.29467-1-jcline@redhat.com


Signed-off-by: Jeremy Cline <jcline@redhat.com>
Acked-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>