contrib: import the relnotes script from the release-tools repo I figure this makes more sense to keep in the main repo, as it's a guide of sorts on how to use the releasenotes extension in the presence of commits that don't get relnotes annotations. Ported to Python 3, cleaned up some logic in a few places, but for the most part it's what we've been using for years. Differential Revision: https://phab.mercurial-scm.org/D4291