A heroku buildpack for setting the ssh private key as part of the application build. It's meant to be used with heroku-buildpack-multi, before other buildpacks which require the key to be present, like installing private repositories from gitlab.
Upload the private key to heroku.
heroku config:set SSH_KEY=$(cat ~/.ssh/id_rsa)
Add a .buildpacks file (used by heroku-buildpack-multi) which contains this and the default python buildpack.
https://github.com/anewbigging/ssh-private-key-buildpack.git
https://github.com/heroku/python
Now as long as the public key is present on gitlab and the user has the correct permissions, it's possible to install from private gitlab repositories.