bpo-34218: Fix a leak in _elementtree.c introduced in GH-6769. (GH-8460)

diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 1500a6d..02781d5 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -3178,6 +3178,7 @@
                 "The doctype() method of XMLParser is ignored.  "
                 "Define doctype() method on the TreeBuilder target.",
                 1);
+        Py_DECREF(res);
     }
 
     Py_DECREF(doctype_name_obj);