On 21 Sep 2017, 5:00 PM +1000, Stefan van der Walt <stefanv@berkeley.edu>, wrote:

Thanks, I never noticed!  All the lessons were contributed for teaching purposes, so I don't imagine adding a license now would be a problem.  CC-BY is fine, although we should also consider CC-0.  

Sure. How about CC0 + adding a note in the readme, “if you find these tutorials useful, we appreciate mentioning them in any derivative works.”