uv python plugin

[lengau]

What needs to get done

Either a separate plugin or an opt-in configuration option for the Python plugin to use uv instead of standard venv and pip.

Why it needs to get done

We should provide first-class support for multiple Python build systems

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The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-2810.

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[tigarmo]

We currently have two draft PRs for this:

• feat: uv python plugin feat: uv python plugin #695
• [experimental] UV support for Python plugins [experimental] UV support for Python plugins #825

@lengau any thoughts on which approach we should follow?

[lengau]

@tigarmo I think we need both, though in different ways.

1. The uv plugin should set UV_PROJECT_ENVIRONMENT={self.get_venv_directory()} and UV_FROZEN=1 and then use uv sync to set up the venv (or something similar)
2. We should have an option in the plain python plugin to use uv venv and uv pip rather than the system's venv and pip. This could also be something like offering a configuration to set the venv and pip commands.

This request is just for the first though.

[lengau]

Per conversation:

• Set UV_PYTHON to $PARTS_PYTHON_INTERPRETER so we're using the system Python (during the build steps, only if not already set)
• Set UV_PYTHON_PREFERENCE=only-system in the build environment.