except on windows, where gla11y will resort to python's internal xml parser,
which does not provide line numbers.

This allows gla11y to be runnable on all systems.

Change-Id: Ica4eb90f59bddfcefd783fc2ed9c8c27357e7572
Reviewed-on: https://gerrit.libreoffice.org/50115Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>