sphinx-doc#9413 Declare py namespace on document root to generate val…

…id xml

sphinx/builders/xml.py

@@ -71,6 +71,7 @@ def write_doc(self, docname: str, doctree: Node) -> None:
         # work around multiple string % tuple issues in docutils;
         # replace tuples in attribute values with lists
         doctree = doctree.deepcopy()
+        doctree.document.attributes["xmlns:py"] = "https://github.com/sphinx-doc/sphinx/issues/9413"
         for node in doctree.traverse(nodes.Element):
             for att, value in node.attributes.items():
                 if isinstance(value, tuple):