relax nbformat

[Borda]

Type

• Bug fix (non-breaking change which fixes an issue): Fixes #
• New feature (non-breaking change which adds functionality). Resolves #
• Breaking change (fix or feature that would cause existing functionality to not work as expected) Resolves #

Motivation and Context

the nbformat is the only hard pin dependency which is a bit strange (I don't see a reason, but there is pls correct me), which makes a third to find an intersection with other packages
also one of the reasons in the past for pin was resolved already in 5.6.1, see: jupyter/nbformat#306 (comment)

Checklist:

• I have run python util/check_style.py --apply to apply Open3D code style
  to my code.
• This PR changes Open3D behavior or adds new functionality.
  • Both C++ (Doxygen) and Python (Sphinx / Google style) documentation is
    updated accordingly.
  • I have added or updated C++ and / or Python unit tests OR included test
    results (e.g. screenshots or numbers) here.
• I will follow up and update the code if CI fails.
• For fork PRs, I have selected Allow edits from maintainers.

Description

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This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[Borda]

@yxlao @errissa @ssheorey @benjaminum could you pls check and approve running CI 🐰

[benjaminum]

Reviewable status: 0 of 1 files reviewed, all discussions resolved

[Borda]

@benjaminum seems that the only failed job failed to start so not even started, could you pls re-run it 🐿️