Upgrade bundled Expat to 2.8.3 (e.g. for the fix to CVE-2026-72522) #155558

Please see blog post https://blog.hartwork.org/posts/expat-2-8-3-released/ for an overview and the change log at https://github.com/libexpat/libexpat/blob/R_2_8_3/expat/Changes for details. Affects all alive branches of Python. Thank you!

Related: #152216 (predecessor for Expat 2.8.2)

CC @StanFromIreland

• CVE-2026-72522

Linked PRs

• gh-155558: Update bundled Expat to 2.8.3 #155560