The extra removal left CI, the Docker image and the nix package still
requesting `hindsight`. Once the extra is gone, `--extra hindsight` and
extraDependencyGroups = [ "hindsight" ] ask for something that no longer
exists. Drop them the same way 73c598e319 originally did: remove it from
the CI extras lists, the Docker sealed-venv build and the nix default
groups, and point the nix examples/check at honcho. Also remove the
stray blank line left in the exclude-newer table.
3aa215fdc9