Update to `jupyterlite-core==0.1.0b20`

[jtpio]

References

Update to the latest jupyterlite-core: https://github.com/jupyterlite/jupyterlite/releases/tag/v0.1.0b20

Which does not include the JavaScript kernel by default anymore: https://github.com/jupyterlite/jupyterlite/pull/1013

Code changes

• [x] Update to jupyterlite-core==0.1.0b20
• [x] Require Python 3.8

User-facing changes

The JavaScript kernel should not be available by default on the demo site in the docs.

Backwards-incompatible changes

None

[github-actions[bot]]

:point_left: Try it on ReadTheDocs

[jtpio]

Looking good: