This is a requirement of ipyparallel (& most IPython/Jupyter packages), so we were already installing it. There was a new major version recently, which Michał found broke pycalibration. I've pinned the latest 4.x version to avoid this.
I also fixed the specific breakage we saw (abbreviating the --daemonize flag to --daemon no longer seems to work). I haven't looked to see if traitlets 5 breaks other things.