See googleapis/synthtool#1916

noxfile.py

@@ -182,7 +182,19 @@ def docs(session):
 
     session.install("-e", ".[tracing]")
     session.install(
-        "sphinx==4.0.1", "alabaster", "recommonmark", "django==2.2"
+        # 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",
+        "alabaster",
+        "recommonmark",
+        "django==2.2"
     )
 
     shutil.rmtree(os.path.join("docs", "_build"), ignore_errors=True)
@@ -207,6 +219,15 @@ def docfx(session):
 
     session.install("-e", ".[tracing]")
     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",
         "gcp-sphinx-docfx-yaml",
         "alabaster",
         "recommonmark",