`ts_utils` should explicitly define a build-system

[Avasam]

uv pip install -r requirements-tests.txt
warning: `/home/runner/work/typeshed/typeshed/lib` does not appear to be a Python project, as the `pyproject.toml` does not include a `[build-system]` table, and neither `setup.py` nor `setup.cfg` are present in the directory

with uv 0.6.5 (bcbcd0a1e 2025-03-06)

Could be a good occasion to try out a lightweight build system other than the default of setuptools for our local lib

Note that if it was already built, you won't get the warning, you must first run:

uv pip uninstall ts_utils
uv cache clean