AHH, thank you! /var/log/dist-upgrade/main.log explains why! It's indeed another instance of a bug I already saw when upgrading another system [1] and couldn't figure out -- there's a missing dependency in some Ubuntu package:

  sudo apt-get install python-distro-info python3-distro-info

EDIT: Do yourself a favor and don't upgrade to 19.04 on WSL. VSCode and google-chrome-stable can't start on it. I had to roll back.

[1] https://bugs.launchpad.net/ubuntu/+source/ubuntu-release-upg...