Bump expat from 2.5.0 -> 2.6.3 (#351)

Author: zanieb

pythonbuild/downloads.py

@@ -89,10 +89,10 @@
         "python_tag": "cp313",
     },
     "expat": {
-        "url": "https://github.com/libexpat/libexpat/releases/download/R_2_5_0/expat-2.5.0.tar.xz",
-        "size": 460560,
-        "sha256": "ef2420f0232c087801abf705e89ae65f6257df6b7931d37846a193ef2e8cdcbe",
-        "version": "2.5.0",
+        "url": "https://github.com/libexpat/libexpat/releases/download/R_2_6_3/expat-2.6.3.tar.xz",
+        "size": 485600,
+        "sha256": "274db254a6979bde5aad404763a704956940e465843f2a9bd9ed7af22e2c0efc",
+        "version": "2.6.3",
         "library_names": ["expat"],
         "licenses": ["MIT"],
         "license_file": "LICENSE.expat.txt",