[3.12] gh-126623: Update libexpat to 2.6.4, make future updates easier (GH-126792) (GH-126797)

gh-126623: Update libexpat to 2.6.4, make future updates easier (GH-126792)

Update libexpat to 2.6.4, make future updates easier.
(cherry picked from commit 3c9996909402fadc98e6ca2a64e75a71a7427352)

Co-authored-by: Seth Michael Larson <seth@python.org>