Almost there, I'm very happy! I have removed the matplotlib dependency from the wonderful @rougier's flocking example code, and added very little #py5 for drawing.
I'm just struggling with "normalizing" the velocity for drawing my triangular boids the same size...
I didn't want to use atan2 to get the heading direction. I thought normalizing velocity would be simpler, but I must be making some silly conceptual mistake. The varying sizes are nice, but I want to understand what I'm doing wrong.