check sphinxcontrib-mermaid for updates

in `requirements.txt`

I have it set to use my fork of a dependency that renders mermaid diagrams in jupyter book
```
git+https://github.com/brownsarahm/sphinxcontrib-mermaid@cdnupdate
```

Need to check for updates to the source project and test if it has any important updates. or if it is fixed so that we can import from pipy instead of my fork. 