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.

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Related Fly.io community and GitHub links

#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.

[kcmartin]

Sorry for the long silence here, and thanks for the thorough work. The direction is right, but we don't have the capacity right now to switch the recommended tooling across the Python docs and support it properly. Closing this for now, with #1844 open to track making uv the default when we can.