[3.12] gh-117187: Fix XML tests for vanilla Expat <2.6.0 (GH-117203) …

…(GH-117244)

gh-117187: Fix XML tests for vanilla Expat <2.6.0 (GH-117203)

This fixes XML unittest fallout from the python/cpython#115398 security fix.  When configured using `--with-system-expat` on systems with older pre 2.6.0 versions of libexpat, our unittests were failing.

* sax|etree: Simplify Expat version guard where simplifiable

Idea by Matěj Cepl

* sax|etree: Fix reparse deferral tests for vanilla Expat <2.6.0

This *does not fix* the case of distros with an older version of libexpat with the 2.6.0 feature backported as a security fix.  (Ubuntu is a known example of this with its libexpat1 2.5.0-2ubunutu0.1 package)
(cherry picked from commit 9f74e86c78853c101a23e938f8e32ea838d8f62e)

Co-authored-by: Sebastian Pipping <[email protected]>

CPython-sync-commit-latest: 1c72265a31eaf8724a5dd16391b951366460b64e