gh-: Ensure that Elements have the ownerDocument attribute

Element uses __slots__, which means that the attribute is not inherited
from the superclass.
All other slots (except _localName) are set in __init__; not including
ownerDocument was an oversight.

Note that creating Element directly is not supported; you're supposed
to use Document APIs. All supported ways of creating elements do set
ownerDocument.
However, many projects ignore the documentation.

Author: encukou

Lib/xml/dom/minidom.py

@@ -689,6 +689,7 @@ class Element(Node):
 
     def __init__(self, tagName, namespaceURI=EMPTY_NAMESPACE, prefix=None,
                  localName=None):
+        self.ownerDocument = None
         self.parentNode = None
         self.tagName = self.nodeName = tagName
         self.prefix = prefix