update file headers with license information (SPDX)
diff --git a/serial/urlhandler/protocol_loop.py b/serial/urlhandler/protocol_loop.py
index 6883b4c..756704b 100644
--- a/serial/urlhandler/protocol_loop.py
+++ b/serial/urlhandler/protocol_loop.py
@@ -9,7 +9,8 @@
# and it was so easy to implement ;-)
#
# (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
#
# URL format: loop://[option[/option...]]
# options: