[3.6] bpo-44394: Update libexpat copy to 2.4.1 (GH-26945) (GH-28042) (GH-28080)

Update the vendored copy of libexpat to 2.4.1 (from 2.2.8) to get the
fix for the CVE-2013-0340 "Billion Laughs" vulnerability. This copy
is most used on Windows and macOS.

Co-authored-by: Victor Stinner <vstinner@python.org>

Co-authored-by: Łukasz Langa <lukasz@langa.pl>.
(cherry picked from commit 3fc5d84046ddbd66abac5b598956ea34605a4e5d)