Create a script to find upstream dependencies

I'm getting an `TypeError: unhashable type: 'dict'` here. I believe that the requirements need to be `flattened` first, since the could be:

* a single task
* a list of tasks
* a dictionary of tasks

Probably doing it like [this](https://github.com/spotify/luigi/blob/cd6e89f3d17fa8e3ff285289a73ec08ee2a2396c/luigi/task.py#L462) would suffice.