Move builds to pyproject.toml while keeping setuptools in place

To proceed with building the system, we should simplify the process. However, we want to avoid introducing new abstractions like `poetry`. Therefore, we should consider adopting pyproject.toml.

Start by inspecting the repository’s existing setuptools build configuration and current build commands, then review the pyproject.toml packaging format. Keep setuptools in place while moving the build configuration into pyproject.toml; done means the system still builds successfully without introducing Poetry or another new abstraction.

Written by the indexing model from the issue text.