Migrate from lxml to another XML parsing library

[sami-m-g]

We are using defusedxml as a secure layer over lxml. However, a DeprecationWarning currently raises when running tests:

DeprecationWarning: defusedxml.lxml is no longer supported and will be removed in a future release.
    from defusedxml.lxml import parse as parse_xml

defusedxml last release was in 2021 so I am not really worried, and this should be a low priority task.
We can move to one of the supported libraries here: https://pypi.org/project/defusedxml/.