test-lowest-version should check for requirements.txt

Currently test-lowest-version will only look for `pyproject.toml` files for dependency lists. It should also check for the existance of a `requirements.txt` file. If one exists, it should be used along with pyproject.toml. 

Hopefully `uv` will know what to do if identical dependencies are listed in both places. Or worse, if different versions of dependencies are pinned in both places. 