Upgrade bundled Expat to 2.8.1 (e.g. for the fix to CVE-2026-45186)

[hartwork]

Please see blog post https://blog.hartwork.org/posts/expat-2-8-1-released/ for an overview and the change log at https://github.com/libexpat/libexpat/blob/R_2_8_1/expat/Changes for details. Affects all alive branches of Python. Thank you!

Related: #149017 (predecessor for Expat 2.8.0)

CC @StanFromIreland

CVE-2026-45186

Linked PRs

• gh-149698: Update bundled expat to 2.8.1 #149699
• [3.15] gh-149698: Update bundled expat to 2.8.1 (GH-149699) #149812
• [3.14] gh-149698: Update bundled expat to 2.8.1 (GH-149699) #149813