chore: Replace CHANGELOG generator

[paleolimbot]

Closes #310.

Basically cz ch was constantly doing the wrong thing...for the last two releases I had to do quite a lot of manual editing after it was run. The Python script that replaces it:

• Can run more than once (replaces the target version's updates if commits were added)
• Groups components (e.g., so I can copy/paste the R NEWS more readily)
• Knows about when nanoarrow considers the last version "released"

[lidavidm]

Hmm, ADBC might want to borrow this too.

[assignUser]

Nice, looks like a neat addition, thanks for adding the tests. At the moment the tests are not run in CI right? (or are they part of pre-commit?)

[paleolimbot]

Thanks for taking a look!

At the moment the tests are not run in CI right?

They aren't, but this script also doesn't see many or any changes and is run once every few months. The hard part is writing them!