3.x Release

[Zsailer]

#384 prompts a major release, since it introduces backwards incompatible support for Jupyter Server.

I've created a new branch for the 2.x line, 2.5.x, which we will maintain for legacy sake for (at a minimum) a year.

The main branch will be the latest, 3.x, release.

I can manage the release. I just need rights to the PyPI package. I'll work on getting Jupyter Releaser setup.

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[blink1073]

• Update packaging
• Add autoformatters
• Add pre-commit config
• Update GitHub Actions
• Add releaser config
• Make release

[blink1073]

[blink1073]

I'll cut an 3.0.0rc0 tomorrow.

[blink1073]

https://github.com/jupyter-server/kernel_gateway/releases/tag/v3.0.0rc1.

We can cut a final release next week.