checkpatch: scripts/spdxcheck.py now requires python3

Since commit d0259c42ab ("spdxcheck.py: Use Python 3"), spdxcheck.py
explicitly expects to run as python3 script.  If "python" still points to
python v2.7 and the script is executed with "python scripts/spdxcheck.py",
the following error may be seen even if git-python is installed for
python3.

Traceback (most recent call last):
  File "scripts/spdxcheck.py", line 10, in <module>
    import git
ImportError: No module named git

To fix the problem, check for the existence of python3, check if
the script is executable and not just for its existence, and execute
it directly.

Link: https://lkml.kernel.org/r/20210505211720.447111-1-linux@roeck-us.net
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: Joe Perches <joe@perches.com>
Cc: Bert Vermeulen <bert@biot.com>
Cc: Dwaipayan Ray <dwaipayanray1@gmail.com>
Cc: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

scripts/checkpatch.pl

@@ -1084,10 +1084,10 @@ sub is_maintained_obsolete {
 sub is_SPDX_License_valid {
 	my ($license) = @_;
 
-	return 1 if (!$tree || which("python") eq "" || !(-e "$root/scripts/spdxcheck.py") || !(-e "$gitroot"));
+	return 1 if (!$tree || which("python3") eq "" || !(-x "$root/scripts/spdxcheck.py") || !(-e "$gitroot"));
 
 	my $root_path = abs_path($root);
-	my $status = `cd "$root_path"; echo "$license" | python scripts/spdxcheck.py -`;
+	my $status = `cd "$root_path"; echo "$license" | scripts/spdxcheck.py -`;
 	return 0 if ($status ne "");
 	return 1;
 }