warn that dask-lightgbm is deprecated

[jameslamb]

Based on my proposal in #29 (comment), I'd like to propose that dask-lightgbm have one more release, and that that release raise a warning on import informing users that the project is deprecated and that it has been merged into lightgbm.

I think this is preferable to just archiving the repo, because it means anyone upgrading dask-lightgbm in automation pip install-ing it based on a Stack Overflow answer or blog post will get a warning nudging them towards lightgbm.

As of the changes in this PR, the following warning will be shown once per session, when you import this library.

python -c "import dask_lightgbm"

/Users/jlamb/repos/dask-lightgbm/dask_lightgbm/core.py:32: UserWarning: You are using the final release of dask-lightgbm. dask-lightgbm functionality has been merged into lightgbm. Consider switching to lightgbm.

cc @jsignell @jrbourbeau

Thanks for your time and consideration.

[jsignell]

Thanks for opening this @jameslamb! Are the failing tests being handled separately?

[jameslamb]

Those same test failures look like they exist on the main branch so I don't think they're related to this PR: https://github.com/dask/dask-lightgbm/runs/1714351507

[jrbourbeau]

Thanks @jameslamb! I'll open a separate issue about releasing