bpo-45618: Fix documentation build by pinning Docutils version (GH-29230)

Author: Maciej Olko

Doc/requirements.txt

@@ -4,6 +4,10 @@
 # won't suddenly cause build failures. Updating the version is fine as long
 # as no warnings are raised by doing so.
 sphinx==3.2.1
+# Docutils version is pinned to a version compatible with the Sphinx
+# version being used. It can be removed after bumping Sphinx version to at
+# least 3.5.4.
+docutils==0.17.1
 
 blurb