See googleapis/synthtool#1916

noxfile.py

@@ -88,7 +88,19 @@ def unitupb(session):
 def docs(session):
  """Build the docs."""
 
- session.install("sphinx==4.2.0", "sphinx_rtd_theme")
+ session.install(
+ # We need to pin to specific versions of the `sphinxcontrib-*` packages
+ # which still support sphinx 4.x.
+ # See https://github.com/googleapis/sphinx-docfx-yaml/issues/344
+ # and https://github.com/googleapis/sphinx-docfx-yaml/issues/345.
+ "sphinxcontrib-applehelp==1.0.4",
+ "sphinxcontrib-devhelp==1.0.2",
+ "sphinxcontrib-htmlhelp==2.0.1",
+ "sphinxcontrib-qthelp==1.0.3",
+ "sphinxcontrib-serializinghtml==1.1.5",
+ "sphinx==4.5.0",
+ "sphinx_rtd_theme",
+ )
  session.install(".")
 
  # Build the docs!