failing RTD build

[dcherian]

The RTD build is failing on all PRs with

Sphinx parallel build error:
nbsphinx.NotebookError: UndefinedError in examples/ERA5-GRIB-example.ipynb:
'nbformat.notebooknode.NotebookNode object' has no attribute 'tags'

[keewis]

the cause is a jinja update (it doesn't interact well with nbsphinx), pinning to jinja<3.0 should fix it