Use a non-experimental repostiory as default

[vlipiainen]

Context

• Binderhub needs a github repostiory to create an environment for interactive code. The default, as defined here, is jupyter-stacks-datascience.
• The repostiory above states that "🛑🛑This is an experimental repository. It might change or be deleted without notice!🛑🛑". This means that sphinx-thebe might stop working without warning.

Proposal

There are two main directions for a fix:

1. Make the jupyter-stacks-datascience repository more stable
2. Use some other default, such as https://github.com/binder-examples/requirements

Constraints & discussion

• We might not be able to affect the experimental status of jupyter-stacks-datascience
• I'm not sure what I picked above is a good default.

Tasks and updates

• Decide between the two solutions
• If changing the default, pick the new default
• Make the small code change

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