Fix unintended behavior change in elementtree introduced in #114269

[Prometheus3375]

Bug report

Bug description:

As it was discussed here, after merging of #114269 it.root is no longer None once an iterator is created. There was no intention to change this, it should be reverted.

Meanwhile, I think it will be good to add test for root attribute.

• Right after function returned iterator: it.root must be None.
• After iterator exhaustion: it.root must be an instance of xml.etree.ElementTree.Element.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Other

Linked PRs

• gh-114737: Revert change to ElementTree.iterparse "root" attribute #114755
• [3.12] gh-114737: Revert change to ElementTree.iterparse "root" attribute (GH-114755) #114798
• [3.11] gh-114737: Revert change to ElementTree.iterparse "root" attribute (GH-114755) #114799