Is there a reason why you can't just use:

    if build.with? "python3"
      depends_on "shiboken" => "with-python3"
    else
      depends_on "shiboken"
    end 

The original `shiboken` formula did not have a `--without-python` option. I imagine it just used the first `python` on the `$PATH`. I don't think it's necessary here.