DM-46777: sphinx < 8.1.0

[Sphinx
8.1.0](sphinx-doc/sphinx@v8.0.2...v8.1.0)
contains [a
commit](https://github.com/sphinx-doc/sphinx/pull/12762/files#diff-a4c6bf1492ef480b94af82c988f64ca56fa256fab0ed043a5ad3d4043f89a645L14)
that removes the `ExtensionError` export from the `sphinx.util` package.
This currently breaks the
[sphinxcontrib-mermaid](https://github.com/mgaitan/sphinxcontrib-mermaid)
dependency:

```
File
"/home/danfuchs/src/safir/.nox/docs-clean/lib/python3.12/site-packages/sphinxcontrib/autoclassdiag.py",
line 3, in <module>    from sphinx.util import ExtensionError,
import_objectImportError: cannot import name 'ExtensionError' from
'sphinx.util'
(/home/danfuchs/src/safir/.nox/docs-clean/lib/python3.12/site-packages/sphinx/util/__init__.py)
```

Pin to < 8.1.0 for now until this gets fixed in either sphinx or
sphinxcontrib-mermaid (and any other packages that we use that might
also be broken)

pyproject.toml

@@ -26,7 +26,7 @@ requires-python = ">=3.11"
 dynamic = ["version"]
 dependencies = [
     "docutils>=0.20",  # solves an extra div bug with the bibliography directive
-    "Sphinx>=7",  # Consistent docutils constraint
+    "Sphinx>=7,<8.1.0",  # Consistent docutils constraint, mermaid compatibility
     "PyYAML",
     "GitPython",
     "requests",