RPI.GPIO should be optional

[kwavnet]

When I attempt to install on an embedded device

luma.core/setup.cfg

Line 36 in caeaae5

RPI.GPIO; platform_system=="Linux"

fails to install for different reasons.

But if I manually install all of the dependencies myself, the actual lib (in this case luma.led_matrix) still works, as apparently RPI.GPIO is only needed if GPIOs are used, but not with

serial = spi(port=0, device=0, gpio=noop())
device = max7219(serial, width=8, height=8)

Wouldn't it be better to turn RPI.GPIO into an extra dependency, like described here https://setuptools.pypa.io/en/latest/userguide/dependency_management.html#optional-dependencies ?