Broken with Sphinx 8.1.0

[jacobtomlinson]

As of the sphinx 8.1.0 release we are now seeing build failures that seem to be coming from this plugin.

Running Sphinx v8.1.0
loading translations [en]... done

Extension error:
Could not import extension sphinxcontrib.mermaid (exception: cannot import name 'ExtensionError' from 'sphinx.util' (.../miniforge3/envs/deployment-docs-dev/lib/python3.12/site-packages/sphinx/util/__init__.py))
make: *** [dirhtml] Error 2

[Paebbels]

It looks like Sphinx 8.1.x moved ExtensionError from sphinx.util to sphinx.errors, but according to #161 (comment), it was moved long time ago and sphinxcontrib-mermaid uses a long outdated API.