Fix to keep working via MyBinder's NbClassic by pinning to old ipywidgets

[fomightez]

I found ipywidgets needs to be pinned to keep it working in NbClassic served by MyBinder for now. (In reply to this StackOverflow post.)

My fork where it is restored to working now is at https://github.com/fomightez/mobilechelonian .
I pinned to ipywidgets 7.6.5 in environment.yml.
Pinning old ipywidgets (based on here and here) restores it to work in NbClassic again.

Launch my fork with the badge below:

Then compare that to what you get from clicking here, which launches the current version in the repo. You'll see you cannot display the Turtle window there (you get something that looks like what is shown image here), and when you check the ipywidgets version there via %pip list, it is 8.1.5

[takluyver]

Thanks for figuring that out! Do you want to make a pull request? It may as well be in a working state; I'm not likely to get time to do anything cleverer any time soon.

[fomightez]

They updated MyBinder just a few hours ago to default to Notebook 7 now, see here. So that means what was working yesterday stopped working. I'll need more time to work out restoring it again.

Right now I was changing the launch badges to target my repo. I'll undo that once it is working again, and make a pull request.