Allow other runtimes

We would need a [shim](https://github.com/apex/up/blob/master/internal/shim/index.js) per-language, but it basically just reads lines which is easier in Python etc than Node anyway so it shouldn't be too difficult to maintain.

- [ ] auto-select `runtime` config based on the inference we already have
- [ ] add python shim and default runtime to python3.6