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

This fixes XML unittest fallout from the https://github.com/python/cpython/issues/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)