PyMIDIK is a simple MIDI keyboard emulator for Linux/evdev.
Simply supplying an input device (/dev/input/eventX) as a command line parameter will launch PyMIDIK and create a MIDI output device. However, it is probably easier to use the -o option to connect to an existing MIDI input device:
fluidsynth -a alsa /usr/share/sounds/sf2/FluidR3_GM.sf2
python3 pymidik.py -o FLUID /dev/input/event0
You can use the -l option to see a list of the available input devices. Make sure you have read access to the /dev device nodes; add your user to the input group or use sudo in a pinch.
The default key mapping is ZSXD CVGBHNJM / Q2W3E R5T6Y7U / I9O0P for octaves 3/4/5, which seems to be some sort of standard.