[3.9] gh-126623: Update libexpat to 2.6.4, make future updates easier (GH-126792)#126800

ambv merged 1 commit intopython:3.9python/cpython:3.9from

sethmlarson:backport-3c99969-3.9sethmlarson/cpython:backport-3c99969-3.9Copy head branch name to clipboard