[FIX] Pin sphinx<9 for docs CI compatibility

sphinx-toolbox.utils imports `logger` from `sphinx.ext.autodoc`,
which was removed in Sphinx 9.1.0. sphinx-toolbox 4.1.2 still has
this issue. Pin sphinx<9 until sphinx-toolbox publishes a real fix.

Author: tqchen

pyproject.toml

@@ -75,7 +75,7 @@ docs = [
   "nbconvert",
   "nbsphinx",
   "nbstripout",
-  "sphinx",
+  "sphinx<9",
   "sphinx-autobuild",
   "sphinx-book-theme",
   "sphinx-copybutton",