update file headers with license information (SPDX)
diff --git a/serial/serialposix.py b/serial/serialposix.py
index 2052f74..4db261b 100644
--- a/serial/serialposix.py
+++ b/serial/serialposix.py
@@ -5,7 +5,8 @@
# 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
#
# parts based on code from Grant B. Edwards <grante@visi.com>:
# ftp://ftp.visi.com/users/grante/python/PosixSerial.py