[bug]: PNG compression help text goes to 0 to 9 for compression when 8 is the max

[Fortyseven]

What happened?

This is pretty trivial, but the help text in invoke.py says --png_compression ranges from 0 to 9, when the argument itself is bound to a max of 8.

  --png_compression {0,1,2,3,4,5,6,7,8}, -z {0,1,2,3,4,5,6,7,8}
                        level of PNG compression, from 0 (none) to 9
                        (maximum). Default is 6.

Current as of 0439b51a