Replace Poetry with Setuptools

Poetry is fine but is another dependency and point of pause for beginners.

We can use [setuptools](https://setuptools.pypa.io/en/latest/) and a default. This moves us to the traditional `venv + pip` workflow that is still most common.

Start by locating the project's Poetry configuration and the current dependency and installation workflow. Compare the existing setup with a setuptools-based venv and pip workflow, then verify that the project can be installed and its dependencies used without Poetry.

Written by the indexing model from the issue text.