binderhub image: refreeze requirements.txt

[jupyterhub-bot]

The binderhub image's requirements.txt has been refrozen based on requirements.in.

[minrk]

refreeze isn't running on a schedule because of the != 'schedule' condition here. I'm not sure if that's on purpose?

The comment doesn't reflect that the schedule of the workflow is monthly, not daily.

[manics]

I think the condition should be github.repository == 'jupyterhub/binderhub' || github.event_name != 'schedule' to prevent scheduled workflows running on forks.

[rgaiacs]

My understanding is that this pull request is out of date after https://github.com/jupyterhub/binderhub/pull/2004.