Sorry, I was still in the middle of writing my announcement on the topic.
You can opt-in to the old interface (aka ‘classic’ notebook) by specifying a url to open instead of a file. In your link, that would be /notebooks/les_variables/bases_python.ipynb. Full link: https://mybinder.org/v2/gh/fontainedeseaux/NSI_premiere/HEAD?urlpath=%2Fnotebooks%2Fles_variables%2Fbases_python.ipynb