support python=3.10

[keewis]

• Passes pre-commit run --all-files
• User visible changes (including notable bug fixes) are documented in whats-new.rst

[keewis]

It seems nbformat dropped the dependency on ipython_genutils, while nbconvert (which is also using it) relied on the implicit dependency. I will add ipython_genutils to the docs requirements for now, but this should be removed once there's a new nbconvert release.