[3.12] gh-126037: fix UAF in `xml.etree.ElementTree.Element.find*` when concurrent mutations happen (#127964) (#131932)

gh-126037: fix UAF in `xml.etree.ElementTree.Element.find*` when concurrent mutations happen (#127964)

We fix a use-after-free in the `find`, `findtext` and `findall` methods of `xml.etree.ElementTree.Element`
objects that can be triggered when the tag to find implements an `__eq__` method that mutates the
element being queried.

(cherry picked from commit c57623c221d46daeaedfbf2b32d041fde0c882de)