When running a sphinx build in parallel, get warning that this extension isn't safe

sphinx-external-toc recently added sphinx-multitoc-numbering as a dependency. 
Now we get a warning with our quick parallel builds:

```
WARNING: the sphinx_multitoc_numbering extension does not declare if it is safe for parallel reading, assuming it isn't - please ask the extension author to check and make it explicit
WARNING: doing serial read
```