I tried this out today and figured out how to run a custom Python application there. You only need two files in a GitHub repo for that: a requirements.txt listing your dependencies and a Procfile that starts a process listening on port $PORT and IP 0.0.0.0.
My Procfile looks like this:
web: datasette . -h 0.0.0.0 -p $PORT
Full notes here: https://til.simonwillison.net/til/til/digitalocean_datasette... replies(1):