As others say, it's headless-gl, and that in turn seems to be due to node-gyp. However the good news seems this is probably more a case of documentation inerti

As others say, it's headless-gl, and that in turn seems to be due to node-gyp. However the good news seems this is probably more a case of documentation inertia than actual requirements: it looks like you actually can install a version of node-gyp that works with Python 3.x and then in turn should be able to get headless-gl working too, which then let's brain.js work with a non -EOL version of Python