The Expat parser is vendored from https://github.com/libexpat/libexpat.
See moz.yaml for the current in-tree version.

To update run

$ ./mach vendor parser/expat/moz.yaml --patch-mode=none
$ hg commit -m "Bug X - Update to Expat x.x.x."
$ ./mach vendor parser/expat/moz.yaml --patch-mode=only
$ hg commit -m "Bug X - Apply local patches to Expat x.x.x."