Fly launch is slightly friendlier to `python` users

We’re slowly catching on that this python things is not blowing over, so we’ve added a bit more support for python users, specifically those who use poetry; if you launch a fastapi, flask, or streamlit app and you manage your dependencies with poetry, fly launch will work out of the box; it will detect your app and build a slim image.

This comes with its own set of docs and fly apps.

:heart_hands: :snake: