romtool fails to add kickety split

[reinauer]

$ romtool build -o ../amiga600.rom -t kick -k -s 512 index.txt 
Traceback (most recent call last):
  File "/home/stepan/.local/bin/romtool", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/home/stepan/.local/lib/python3.11/site-packages/amitools/tools/romtool.py", line 740, in main
    result = opts.cmd(opts)
             ^^^^^^^^^^^^^^
  File "/home/stepan/.local/lib/python3.11/site-packages/amitools/tools/romtool.py", line 202, in do_build_cmd
    rom_data = rb.build_rom()
               ^^^^^^^^^^^^^^
  File "/home/stepan/.local/lib/python3.11/site-packages/amitools/rom/rombuilder.py", line 193, in build_rom
    rom_data = RomBuilder.build_rom(self)
               ^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/home/stepan/.local/lib/python3.11/site-packages/amitools/rom/rombuilder.py", line 148, in build_rom
    rom_data[off : off + n] = mod.get_data(addr)
                              ^^^^^^^^^^^^^^^^^^
  File "/home/stepan/.local/lib/python3.11/site-packages/amitools/rom/rombuilder.py", line 57, in get_data
    return data + hdr
           ~~~~~^~~~~
TypeError: can only concatenate str (not "bytes") to str