.inx validation fails with libxml 2.15

libxml 2.15 disables Schematron by default and plans to remove Relax NG support.

As a result, the Schematron tests will fail with default libxml build configuration:

ERROR inkex/tester/test_inx_file.py - lxml.etree.SchematronError: lxml.etree was compiled without Schematron support.
ERROR tests/test_inkex_inx.py - lxml.etree.SchematronError: lxml.etree was compiled without Schematron support.

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