Revert PR #378 (Simplify NotebookNotary._data_dir_default)

[krassowski]

Reverts https://github.com/jupyter/nbformat/pull/378 to close https://github.com/jupyter/nbformat/issues/396.

This is because the breakage was also reported in the wild, see:

• https://github.com/voila-dashboards/voila/issues/1450
• https://github.com/jupyterlab/jupyterlab/issues/15985

[ivanov]

I'd like to keep https://github.com/jupyter/nbformat/issues/396 open to think through alternatives or improvements

Nevermind that last part, the only remaining issue belongs in jupyter_core, so I opened https://github.com/jupyter/jupyter_core/issues/397

[krassowski]

Thanks for the merge! Another downstream affected: https://github.com/widgetti/ipyvuetify/pull/301

@blink1073 a new patch release would be highly valued (once you have time to look at it)

[blink1073]

Done!

[krassowski]

Thank you!