Not installable on MacOS for development

[Jonty]

The hard spidev requirement in the setup.py script means this isn't installable on MacOS as it is linux-only.

It isn't needed for Mock simulation and development, and I think it should probably only be required when installing rpi-gpio-output or rpi extras.

[Gadgetoid]

I'm wary of trying to support installation on non-Pi-Linux operating systems, since any accommodation we make to make macOS or Windows easier adds friction to the primary use case- on device!

As such, rpi-gpio-output has been dropped as part of the switch to gpiod, and virtualenvs, since it's basically untenable- making a fundamental requirement optional is just silly.

I don't know if there's a way around this, but I generally assume anyone using the simulator can just clone the repository and edit those deps out.