update file headers with license information (SPDX)
diff --git a/serial/serialutil.py b/serial/serialutil.py
index 4d2c7e0..a71bd37 100644
--- a/serial/serialutil.py
+++ b/serial/serialutil.py
@@ -3,7 +3,8 @@
 # see __init__.py
 #
 # (C) 2001-2015 Chris Liechti <cliechti@gmx.net>
-# this is distributed under a free software license, see license.txt
+#
+# SPDX-License-Identifier:    BSD-3-Clause
 
 import io