It’s just occurred to me that #12088 pulled in the “packaging” package (~=21.3). I pulled in the newest version I had at the time.
I only use it for packaging.requirements.Requirements. Which was added in packaging 16.1: https://github.com/pypa/packaging/releases/tag/16.1https://pkgs.org/download/python3-packaging suggests that the oldest version we care about is 17.1 in Ubuntu Bionic. So I think with this bound we're hunky dory.
David Robertson