Copyright date bump and apply SPDX tags.

The tags are shorthand for the license of each file and avoid
copying the full license text into each one (and avoids having
to manually update the dates in each one...).

scripts/music-duplicates.py

@@ -1,4 +1,5 @@
 #!/usr/bin/env python
+# SPDX-License-Identifier: BSD-3-Clause
 #
 # Find duplicated music tracks and create a summary report of music
 # that the project needs.