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?