Fixed pip install error

[jamesjun]

Fixed pip install error by adding [tool.hatch.build.targets.wheel] section in pyproject.toml.

To reproduce the error run pip install -e ..

[dlqqq]

@jamesjun Thanks for this PR! We appreciate others taking their time to contribute. I want to highlight two points:

1. Contributors and developers should run jlpm dev-install to install Jupyter AI locally in a dev environment. They should not try to run pip install . on the "top-level package" at the root of the repository, since this is a monorepo containing multiple packages. The top-level package doesn't really do anything yet, and exists mainly to force our build & CI tools to work.
2. I'm working on a PR to use the top-level package to gather shared dev & test dependencies. However, this won't automatically install jupyter-ai and jupyter-ai-magics. It's meant to be installed via jlpm dev-install. See: WIP: Gather shared Python dependencies #544.

Because of these two points, I'm going to tentatively close the PR. Please reply with feedback if you don't agree with this, and I will address it. Thank you!