disable installation on non-osx platforms.
There are regular questions from users that try to use py2app on Windows or Linux to build an application bundle.
That's an unsupported configuration that fails with a rather confusing message.
Check that the CLASSIFIERS list mentions that this is an OSX only library
Add hooks that prints a nice error message when a user tries to install on a platform that isn't OSX (simular to how pyobjc-framework-* checks for supported systems)