CI: MusicXML tests fail to load XML Schema file

[igneus]

So far I haven't experienced this issue when running the tests on my workstation.

Example pytest output from https://github.com/frescobaldi/python-ly/actions/runs/19602803582/job/56136729645?pr=181 :

[...]
________________________________ test_glissando ________________________________

    def test_glissando():
>       compare_output('glissando')

tests/test_xml.py:16: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 
tests/test_xml.py:109: in compare_output
    validate_xml(output)
tests/test_xml.py:118: in validate_xml
    xmlschema = etree.XMLSchema(xmlschema_doc)
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

>   ???
E   lxml.etree.XMLSchemaParseError: attribute use (unknown), attribute 'ref': The QName value '{http://www.w3.org/XML/1998/namespace}lang' does not resolve to a(n) attribute declaration., line 1810

src/lxml/xmlschema.pxi:90: XMLSchemaParseError
[...]

[fedelibre]

Fixed by #189