replace seticon Python script with AppleScript

Updated makefile to replace the Python3 script for setting the binary icon with an AppleScript version shamelessly copied from [this answer](https://apple.stackexchange.com/a/373574). This also ensures the makefile will run in the future as per the macOS 10.15 release notes: > Scripting language runtimes such as Python, Ruby, and Perl are included in macOS for compatibility with legacy software. Future versions of macOS won’t include scripting language runtimes by default, and might require you to install additional packages. If your software depends on scripting languages, it’s recommended that you bundle the runtime within the app.