Red Hat lawyers concluded
(https://gitlab.com/fedora/legal/fedora-license-data/-/issues/154) that the
license included in
https://github.com/lxml/lxml/blob/master/src/lxml/isoschematron/resources/rng/iso-schematron.rng
is confusing, misworded, and most likely non-Free (distribute-only-for-gratis
problem).

They started to remove the schema file from their project (e.g.,
https://src.fedoraproject.org/rpms/python-lxml/c/9d95f5a04edc386313fa854541971b3af07bcae1?branch=rawhide).

This issue has been reported upstream as
https://bugs.launchpad.net/lxml/+bug/2024343 and they created patch which
allows the test suite to pass even without RNG file in the question.