mkclean: use SPDX license in python script

mkclean/regression/mkcleanreg.py

@@ -1,19 +1,7 @@
 #!/usr/bin/env python3
 
-# ISC License
-#
-# Copyright 2022 Matroska (non-profit organisation)
-#
-# Permission to use, copy, modify, and/or distribute this software for any purpose with or without
-# fee is hereby granted, provided that the above copyright notice and this permission notice appear
-# in all copies.
-#
-# THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS
-# SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE
-# AUTHOR BE LIABLE FOR ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
-# WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN ACTION OF CONTRACT,
-# NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR PERFORMANCE
-# OF THIS SOFTWARE.
+# SPDX-License-Identifier: ISC
+# Copyright (c) 2022, Matroska (non-profit organisation)
 
 import argparse
 import pathlib