python/samba/gp_parse: Fix test errors with python3.8

python/samba/gp_parse: Fix test errors with python3.8

UNEXPECTED(failure): samba.tests.samba_tool.gpo.samba.tests.samba_tool.gpo.GpoCmdTestCase.test_backup_restore_generalize(ad_dc:local)
REASON: Exception: Exception: Traceback (most recent call last):
  File "/tmp/samba-testbase/b28/samba-ad-dc-1/bin/python/samba/tests/samba_tool/gpo.py", line 434, in test_backup_restore_generalize
    self.assertIsNone(has_difference(os.path.join(new_path, 'policy',

This caused because prior to 3.8 minodom.toprettyxml() was sorting the
attribute order, now it preserves the attribute order specified by the user
Signed-off-by: Noel Power <noel.power@suse.com>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>