Fix dependencies for test extra by downgrading to nbdime<4

[amotl]

Problem

Both PRs submitted by Dependabot revealed that something went south.

• build(deps-dev): update pyproject-fmt requirement from <1.5,>=1.3 to >=1.3,<1.6 #21
• build(deps-dev): bump aiohttp from 3.9.0b1 to 3.9.0 #22

Analysis

We discovered that it was about the nbdime package, apparently breaking a little detail with its 4.x release. We submitted a corresponding report to the upstream repository.

• https://pypi.org/project/nbdime/#history
• nbdime 4.x: TypeError: diff() got an unexpected keyword argument 'predicates' jupyter/nbdime#741

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6e5878a) 74.37% compared to head (378e301) 74.37%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #23   +/-   ##
=======================================
  Coverage   74.37%   74.37%           
=======================================
  Files          17       17           
  Lines         562      562           
=======================================
  Hits          418      418           
  Misses        144      144           

Flag	Coverage Δ
main	74.37% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.