Update Python docs to use `uv` instead of `poetry`

[mattt]

Summary of changes

This PR updates the Python guides to use uv for dependency management.

Preview

Related Fly.io community and GitHub links

https://github.com/superfly/docs/issues/1844

Notes

• The "Setting up a Python Environment" guide is mostly the same. Because uv uses venv by default, fewer configuration steps are required.
• The "Multi-stage Builds" guide is a more involved change.
• The poetry_new partial is replaced with a new uv_init partial, and all of its references are updated.
• The ["Getting Started"] guide for Django still uses pip + venv, and is updated only to mention uv as an alternative (as it currently does for Poetry)
• The remaining changes are simple replacements of poetry add → uv add, which does the same thing.