Arg. The ipython install is a little idiosyncratic. Here’s what I had to do to resolve this:
$ pip uninstall ipython
$ pip install "ipython[all]"
The issue is that notebooks have their own set of dependencies, which aren’t installed with pip install ipython. However, having installed ipython, pip doesn’t see the need to add anything if you then try the [all] form.
As mentioned in comments for some shells (e.g. zsh) it’s necessary to escape or quote the square brackets (pip install ipython\[all\] would also work).