Piano Hat Midi on Raspberry Pi 3

no, this has nothing to do with compatibility with Raspberry Pi 3... I tried it on a clean Jessie install and at first got the same issue on a Pi 2.

if you check the setup.py file in the python-midi repo you can see that it's looking for ALSA headers, so just run:

sudo apt-get install libasound2-dev swig

then remove the version you installed previously using:

sudo pip uninstall midi

last, clean rebuild/install python-midi using the setup file.
It should work then.