Pin sphinx extension versions

requirements-docs.txt

@@ -3,4 +3,13 @@ Sphinx==3.0.2
 sphinx-notfound-page==0.4
 # alabaster 0.7.14 dropped support for Sphinx<3.4
 # and Sphinx 3.0.2 bounds alabaster>=0.7,<0.8.
-alabaster==0.7.13
+alabaster==0.7.13
+# Latest versions of these packages support only
+# Sphinx>5, which we are not using. Set explicit pins
+# so we can continue to use Sphinx 3.
+sphinxcontrib-applehelp==1.0.4
+sphinxcontrib-devhelp==1.0.2
+sphinxcontrib-htmlhelp==2.0.1
+sphinxcontrib-jsmath==1.0.1
+sphinxcontrib-qthelp==1.0.3
+sphinxcontrib-serializinghtml==1.1.5