Update the interface for declaring Python download preferences

[zanieb]

The loose consensus is that "fetch" doesn't have much meaning and that a boolean flag makes more sense from the command line.

1. Adds --allow-python-downloads (hidden, default) and --no-python-downloads to the CLI to quickly enable or disable downloads
2. Deprecates --python-fetch in favor of the options from (1)
3. Removes python-fetch in favor of a python-downloads setting
4. Adds a never variant to the enum, allowing even explicit installs to be disabled via the configuration file

Test plan

I tested this with various pyproject.toml-level settings and uv venv --preview --python 3.12.2 and uv python install 3.12.2 with and without the new CLI flags.

[zanieb]

Notably I did not deprecate the python-fetch setting, just removed it. I'm not sure what the pattern is to deprecate a setting yet, nor am I sure it's worth the extra effort.

[zanieb]

I'm a little uncertain of the "never" value, but I do think it has some utility to require explicit opt-in from the CLI to install versions.

[zanieb]

Whoops I checked in my pyproject.toml change, and look, it works https://github.com/astral-sh/uv/actions/runs/10310109626/job/28541254932?pr=5936

[charliermarsh]

Thanks. Am I right that --python-downloads isn't exposed on the command line? So those settings are only available in a configuration file?

I'm wondering it it should be --download-python and --no-download-python to match settings like --compile-bytecode. Are we consistent with that pattern?

[zanieb]

I leaned away from --download-python because it won't force Python to be downloaded, it just allows it if it would be, e.g. --python-preference only-system --download-python would still never download Python.

[zanieb]

Am I right that --python-downloads isn't exposed on the command line? So those settings are only available in a configuration file?

Yes, only a boolean option is available from the command line and the "manual" value is only accessible via the configuration file. I find that a little peculiar, but don't think it'll be problematic in practice.