CI doesn't work for jupyter-book==0.6.5

The circleci falls down with the most recent release of `jupyter-book`, claiming that it cannot find the toc.

```
Couldn't find a Table of Contents file. To auto-generate one, run

jupyter-book toc .
```

But there is a toc. Need to work out why this is, for now `jupyter-book==0.6.4` is the the `requirements.txt`