Fix .python-version lookup

[jku]

Our selftest and Makefile happily find the .python-version in the working directory, but for actual users of the action it's not as easy: The .python-version is actually in the action path.

I'm re-adding 3.10 to test matrix so we can for sure see the python version selection in the logs

[jku]

looks good to me, this is from a test run where "system python" is 3.10:

first the host environment

DEBUG: Python: Python 3.10.11
DEBUG: pip: pip 25.3 from /Library/Frameworks/Python.framework/Versions/3.10/lib/python3.10/site-packages/pip (python 3.10)

then uv reads the python-version (3.14) and sets up the actual runtime:

Using CPython 3.14.0 interpreter at: /opt/homebrew/opt/[email protected]/bin/python3.14
Creating virtual environment at: .action-env
Activate with: source .action-env/bin/activate
Using Python 3.14.0 environment at: .action-env