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...).
Tweak the code used to build the GENMIDI Lump so that it properly
supports Python 3 for build. Tested use cases were:
* Normal build
* a2i-to-sbi script to convert AdTrack2 instruments to SBI format
* dumpgenmidi script to dump the instruments from a GENMIDI lump.
* Running genmidi.py, sbi_file.py, a2i_file.py standalone to print the
contents of files in their respective formats.