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

hghave: move from requiring the PYTHON3 env var to looking for `python3` We're starting to write new utility scripts in Python 3, and I think it's reasonable to expect `python3` to be something we should use if it's present. I wrote the regex to specifically ban Python 3 prior to Python 3.5 because we won't ever support that. I moved the only test that was using $PYTHON3 to just using `python3` directly. Differential Revision: https://phab.mercurial-scm.org/D4290