Have no idea of Python but looking at the example repo and the docs page you linked to, do you have a Procfile?
And just in case, is your entry .py file in the root next to the fly.toml? This is probably not needed with a Procfile not sure tho.
Building a Dockerfile will serve you better over the longer term, as you’ll have more ability to control your build process & deviate from the (narrower) expectations and capabilities of a buildpack.
Here’s a “python and poetry dockerfile” example, we deploy this app to Fly. (Incidentally it also uses nodejs, but you should be able to strip out the parts you don’t need).