Simplify release process with "release-please"

[wetneb]

I recently discovered release-please which eases the release process by automating quite a lot of the tasks that we currently do manually. I would be keen to try out this tool and see if it could work for Wikidata-Toolkit, which has a fairly low traffic.

This would require adopting the "conventional commits" style of commit message:

• fix: which represents bug fixes, and correlates to a SemVer patch.
• feat: which represents a new feature, and correlates to a SemVer minor.
• feat!:, or fix!:, refactor!:, etc., which represent a breaking change (indicated by the !) and will result in a SemVer major.

The migration to this release process would require merging #131, as "release-please" doesn't handle the generation and upload of release artifacts by itself.