Fix build failure for Docutils 0.18.0 (#10597)

CHANGES

@@ -17,6 +17,8 @@ Bugs fixed
 ----------
 
 * #10594: HTML Theme: field term colons are doubled if using Docutils 0.18+
+* #10596: Build failure if Docutils version is 0.18 (not 0.18.1) due
+  to missing ``Node.findall()``
 
 Testing
 --------

sphinx/util/docutils.py

@@ -550,9 +550,9 @@ def unknown_visit(self, node: Node) -> None:
 
 
 # Node.findall() is a new interface to traverse a doctree since docutils-0.18.
-# This applies a patch docutils-0.17 or older to be available Node.findall()
+# This applies a patch to docutils up to 0.18 inclusive to provide Node.findall()
 # method to use it from our codebase.
-if docutils.__version_info__ < (0, 18):
+if docutils.__version_info__ <= (0, 18):
     def findall(self, *args, **kwargs):
         return iter(self.traverse(*args, **kwargs))