WIP: Gather shared Python dependencies

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Gathers dev & test dependencies shared by all Jupyter AI packages into the root pyproject.toml. This helps make the codebase less confusing, as developers do not need to re-declare dependencies in each package pyproject.toml.

I've switched to the default setuptools build backend for the monorepo root package for simplicity (since we don't need a sophisticated build for a dummy package that is never released). I still need to update bump-version.sh to bump the root pyproject.toml.