Have `GithubApi.last_release` ignore prereleases

[duckinator]

At the moment, if you (for example) release v6.0.0b1 then v6.0.0, you have to delete the v6.0.0b1 release first or v6.0.0 won't include all the relevant changes from the last stable version.

If GithubApi.last_release were to ignore prereleases (TODO: determine technical requirements for this), it would avoid this problem entirely. It might be worth renaming it to make it clear it doesn't include prereleases (maybe to last_stable_release?), but I'm not sure it matters as long as it's documented.

[duckinator]

I've found that, for me, every time I do a beta release I later come to the conclusion that it's because I merged something before it was ready, something needed a follow-up PR that wasn't ready for some reason, or I wasn't confident in CI. So it makes more sense to improve automated testing, hold myself to the same code standards I hold outside contributions, and avoid merging multi-part PRs until every part is in place.

Given that, I'm going to close this.

If someone else wants this feature, please feel free to either leave a comment here or open a new issue asking for it. I'm not opposed to it, I just don't want to add features if there's not demand.

[duckinator]

The codebase filters out prereleases elsewhere -- namely bork.github.GithubRelease._get_release_info() -- so I think I'm going to consider this a bug, not a feature request.