Add release checklist

[hugovk]

Checklist for releasing via trusted publishing (https://github.com/python/cherry-picker/pull/94).

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Note: because the last tags weren't created on main (see https://github.com/python/cherry-picker/issues/96), the first time following this, when clicking "Generate release notes", it will pre-fill too much stuff.

So the first time, I suggest to either:

• leave it blank, or
• manually add the new things, or
• generate and remove all except the relevant.

[hugovk]

Let's merge and iterate as needed.