[3.12] gh-114737: Revert change to ElementTree.iterparse "root" attribute (GH-114755) (GH-114798)

Prior to gh-114269, the iterator returned by ElementTree.iterparse was
initialized with the root attribute as None. This restores the previous
behavior.
(cherry picked from commit 66f95ea6a65deff547cab0d312b8c8c8a4cf8beb)

Co-authored-by: Sam Gross <colesbury@gmail.com>