Support .python-version files as input to with python-version

[JacobHayes]

In simple projects, I use a .python-version file to set the version and don't use GHA matrix builds across python versions (ie: no python version ref in workflow file).

I would like to use setup-uv's python installation and particularly the automatic venv activation, but don't want to duplicate the python version in both the .python-version file _and_ the GHA workflow. Right now, I just use setup-uvand then runuv sync --frozenin the next step (which will implicitly install from.python-version`), which works fine but doesn't handle the venv activation for subsequent steps.

The presence of a .python-version file probably shouldn't automatically trigger the python install / venv activation, but perhaps the python-version flag could be extended to support a reference to the file?

[kb-beebucket]

maybe we need

with: 
    sync: true/<specific-extra>/all-extras

and

with:
    activate-venv: true/<path>

in addition to with: python-version which specifies the first two options implicitly.

[rnwolf]

I get the following error:

[Install uv](https://github.com/rnwolf/our-planner/actions/runs/14222047083/job/39852292934#annotation:3:1)
Unexpected input(s) 'python-version-file', valid inputs are ['version', 'pyproject-file', 'uv-file', 'python-version', 'checksum', 'github-token', 'enable-cache', 'cache-dependency-glob', 'cache-suffix', 'cache-local-path', 'prune-cache', 'ignore-nothing-to-cache', 'ignore-empty-workdir', 'tool-dir', 'tool-bin-dir']

This was here I suspect:

      - name: Install uv
        uses: astral-sh/setup-uv@v5
        with:
          python-version-file: ".python-version"
          version: "0.6.11"

And when I setup python environment the first time it was pinned with ".python-version".
As per https://docs.astral.sh/uv/concepts/python-versions/#python-version-files
I don't see a suggestion that we should use "python-version". Would be good to get valid input for this file as well for ".python-version"