Unpin jupyter-book in docs/requirements.txt

[ctuguinay]

Leaving this here so I don't forget. The latest jupyter-book version does not allow us to buildthedocs. I pinned it in this PR #1530. To use the latest version, I found that we need to specify a nodejs version in build tools (https://docs.readthedocs.com/platform/stable/config-file/v2.html#build-tools) and that calling jupyter-book config sphinx docs/source is now an invalid command.

[LOCEANlloydizard]

Hi @ctuguinay, thanks a lot for raising this! I’ve added the nodejs/jupyter-book changes you suggested in a small PR that we can merge easily (which also fixes a minor test issue caused by the recent xarray update). With this RTD fix, we should be back on track!