Make samplerbox.py work with rtmidi-python

[mojca]

I'm having troubles installing rtmidi_python on Windows. After checking, it seems like the project is pretty abandoned. In order to make SamplerBox working on my PC, I had to switch to python-rtmidi instead (and make changes necessary for python 3 compatibility as I didn't want to install python 2.7).

This is a trivial patch that makes this possible.