With https://salsa.debian.org/webmaster-team/webwml/-/merge_requests/737 now merged, python3 support is in https://salsa.debian.org/webmaster-team/webwml/-/merge_requests/740. I'll open an RT ticket to get https://salsa.debian.org/seb/debian.org/-/commit/72fbf295abfd042835ce786344a13bcf8a81148b included so python3-lxml is available on the server side.