Prepare for release of version 5.3.0 of apptools

[mdickinson]

This PR (against the main branch) bumps the version and updates the changelog in preparation for a 5.3.0 release of Apptools. The plan is to tag the merge commit from the eventual squash-merge of this PR.

@flongford Would you have bandwidth to review?

[flongford]

LGTM, though with the disclaimer that I'm not very familiar with the current release process.

Nice to see how the scriv snippets contribute to the release notes!

[mdickinson]

@flongford Thanks for review!

I've just discovered that there are release notes for 5.2.1 on the 5.2.1 tag, but that they weren't ever merged back into the main branch (and they have an oddly incorrect release date). I've done my best to reconcile those with the ones introduced in this PR.

I'm not very familiar with the current release process

There isn't really one, or at least not one that I'd care to imitate. We had a somewhat over-elaborate process for previous releases, and I'm doing something simpler this time around (in particular, since this is a minor release rather than a patch release, I'm releasing straight from the main branch). I'll add some notes on the process to the (currently empty) wiki, for possible re-use next time.

Nice to see how the scriv snippets contribute to the release notes!

Yep, this was our own home-grown approximation to scriv (from a time that predated scrivs existence), introduced by @kitchoi. Now that scriv exists, I think we should probably switch to using that from this point onwards.

[mdickinson]

I'll add some notes on the process to the (currently empty) wiki

Started some notes here: https://github.com/enthought/apptools/wiki/Releasing