[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)

• Doc/library/xml.rst[diff]
• Doc/whatsnew/3.6.rst[diff]
• Misc/NEWS.d/next/Security/2021-06-29-02-45-53.bpo-44394.A220N1.rst[Added - diff]
• Modules/expat/COPYING[diff]
• Modules/expat/ascii.h[diff]
• Modules/expat/asciitab.h[diff]
• Modules/expat/expat.h[diff]
• Modules/expat/expat_external.h[diff]
• Modules/expat/iasciitab.h[diff]
• Modules/expat/internal.h[diff]
• Modules/expat/latin1tab.h[diff]
• Modules/expat/nametab.h[diff]
• Modules/expat/siphash.h[diff]
• Modules/expat/utf8tab.h[diff]
• Modules/expat/winconfig.h[diff]
• Modules/expat/xmlparse.c[diff]
• Modules/expat/xmlrole.c[diff]
• Modules/expat/xmlrole.h[diff]
• Modules/expat/xmltok.c[diff]
• Modules/expat/xmltok.h[diff]
• Modules/expat/xmltok_impl.c[diff]
• Modules/expat/xmltok_impl.h[diff]
• Modules/expat/xmltok_ns.c[diff]