Update XML test for Python 3.8+.

tests/comparators/test_xml.py

@@ -17,6 +17,7 @@
 # You should have received a copy of the GNU General Public License
 # along with diffoscope.  If not, see <https://www.gnu.org/licenses/>.
 
+import sys
 import pytest
 
 from diffoscope.comparators.xml import XMLFile
@@ -46,6 +47,7 @@ def differences(xml_a, xml_b):
     return xml_a.compare(xml_b).details
 
 
+@pytest.mark.skipif(sys.version_info < (3, 8), reason="requires python3.8 or higher")
 def test_diff(differences):
     expected_diff = get_data('test_xml_expected_diff')
     assert differences[0].unified_diff == expected_diff

tests/data/test_xml_expected_diff

 @@ -1,9 +1,9 @@
  <?xml version="1.0" encoding="utf-8"?>
  <note>
-   <style id="night-mode-pro-style" type="text/css"/>
-   <link id="night-mode-pro-link" rel="stylesheet" type="text/css"/>
+   <style type="text/css" id="night-mode-pro-style"/>
+   <link type="text/css" rel="stylesheet" id="night-mode-pro-link"/>
 -  <to>Tove</to>
 -  <from>Jani</from>
 -  <heading>Reminder</heading>