Upgrade bundled Expat to 2.8.4 (e.g. for 4 security fixes)

[hartwork]

Please see blog post https://blog.hartwork.org/posts/expat-2-8-4-released/ for an overview and the change log at https://github.com/libexpat/libexpat/blob/R_2_8_4/expat/Changes for details. Affects all alive branches of Python. Thank you!

Related: #155558 (predecessor for Expat 2.8.3)

CC @StanFromIreland

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• CVE-2026-66046
• CVE-2026-76641
• CVE-2026-76956
• CVE-2026-76957

• gh-156723: Update bundled libexpat to version 2.8.4 #156724
• [3.15] gh-156723: Update bundled libexpat to version 2.8.4 (GH-156724) #156731
• [3.14] gh-156723: Update bundled libexpat to version 2.8.4 (GH-156724) #156732
• [3.13] gh-156723: Update bundled libexpat to version 2.8.4 (GH-156724) #156733
• [3.12] gh-156723: Update bundled libexpat to version 2.8.4 (GH-156724) #156785
• [3.11] gh-156723: Update bundled libexpat to version 2.8.4 (GH-156724) #156786
• [3.10] gh-156723: Update bundled libexpat to version 2.8.4 (GH-156724) #156787