This is a Heroku buildpack for Python apps, powered by pip. It will also install the NPM packages that are defined in the npm_requirements.txt file in the root of the project.
This is pretty hacky. At the moment it reinstalls all the defined npm packages on every deploy. It will also only work if your slug builder already has NPM installed. For dokku, add npm to the packages.txt of the buildstep step.
Probably a better solution would be to use the Heroku multi buildpack.