community/jupyter-nbclient: upgrade to 0.6.7

https://github.com/jupyter/nbclient/releases/tag/v0.6.7

v0.6.6...v0.6.7

• Fix tests for ipywidgets 8 #246 (@frenzymadness)
• [pre-commit.ci] pre-commit autoupdate #236 (@pre-commit-ci)