Add sphinx<9 pin

```
WARNING: while setting up extension sphinx.ext.autodoc.typehints:
extension 'sphinx.ext.autodoc.typehints' has no setup() function; is it
really a Sphinx extension module?
```

Author: hynek

.pre-commit-config.yaml

@@ -4,7 +4,7 @@ ci:
 
 repos:
   - repo: https://github.com/astral-sh/ruff-pre-commit
-    rev: v0.14.10
+    rev: v0.14.13
     hooks:
       - id: ruff-check
         args: [--fix, --exit-non-zero-on-fix]

pyproject.toml

@@ -52,7 +52,7 @@ typing = ["mypy>=1.4", "rich", "twisted"]
 docs = [
     "cogapp",
     "myst-parser",
-    "sphinx",
+    "sphinx<9",
     "sphinx-design",
     "sphinx-copybutton",
     "sphinx-book-theme",