Fix docs build (#8472)

Author: hendrikmakait

docs/requirements.txt

@@ -5,6 +5,13 @@ cloudpickle
 git+https://github.com/dask/dask
 sphinx
 dask-sphinx-theme>=3.0.0
+# FIXME: `sphinxcontrib-*` pins are a workaround until we have sphinx>=5.
+#        See https://github.com/dask/dask-sphinx-theme/issues/68.
+sphinxcontrib-applehelp>=1.0.0,<1.0.7
+sphinxcontrib-devhelp>=1.0.0,<1.0.6
+sphinxcontrib-htmlhelp>=2.0.0,<2.0.5
+sphinxcontrib-serializinghtml>=1.1.0,<1.1.10
+sphinxcontrib-qthelp>=1.0.0,<1.0.7
 sphinx-click
 memray
 sphinx_design