Add uv project

[andersju]

See https://kbse.atlassian.net/browse/LXL-4675

I removed .python-version from .gitignore because consensus seems to be that it should be checked in, to make sure we're all running the same version. (uv init foo creates both foo/.gitignore and foo/.python-version and doesn't git-ignore the latter.) From one of the main uv authors:

The information [.python-version] captures is materially different than requires-python. requires-python just tells you the range of supported versions; meanwhile, teams use .python-version to dictate the version that should be used during development. That’s useful both for libraries and applications.

It also does not need to be a patch version – you can put 3.12 or similar to ensure that everyone is using Python 3.12, rather than anything later or earlier.

(https://discuss.python.org/t/standardize-python-version-file-in-pyproject-toml/86342/18)

Keeping requirements.txt around for now but at some point it should be removed. (...or we just do it right away and remove the pip/manual virtualenv stuff from the README?)