Adopt towncrier to manage CHANGELOG updates

[pradyunsg]

I've missed updating the changelog entries for a release twice now. It's probably a good idea to now update our processes to avoid that mode of failure.

[brettcannon]

There's also scriv (it's what I use to automate the Python Launcher for Unix).

Otherwise GitHub now has automatic release notes generation that we could use instead.

[pradyunsg]

I'm not a 100% sure how we'd use GitHub's generation with https://packaging.pypa.io/en/latest/changelog.html.

Although, I'm also fine with just dropping that in favour of GitHub's releases page.

[brettcannon]

I think there are two options (and you touched on one of them).

One, don't worry about having an accumulated file and just point people at the releases to see what changed (as you suggested).

Two, use https://docs.github.com/en/rest/reference/repos#generate-release-notes-content-for-a-release to automatically update the changelog as part of the release.

I'm personally fine with option 1 since that's the easiest for us and contributors. 😁