[3.12] gh-101438: Avoid reference cycle in ElementTree.iterparse. (GH-114269) (GH-114499)

The iterator returned by ElementTree.iterparse() may hold on to a file
descriptor. The reference cycle prevented prompt clean-up of the file
descriptor if the returned iterator was not exhausted.
(cherry picked from commit ce01ab536f22a3cf095d621f3b3579c1e3567859)

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