archives: do not keep update.xml text in field

Signed-off-by: Hiroshi Miura <miurahr@linux.com>

tests/test_doc_archives.py

@@ -1,4 +1,5 @@
 import os
+from pathlib import Path
 
 import pytest
 
@@ -21,8 +22,7 @@ def setup():
 )
 def test_parse_update_xml(monkeypatch, os_name, version, flavor, datafile):
     def _mock(self, url):
-        with open(os.path.join(os.path.dirname(__file__), "data", datafile), "r") as f:
-            self.update_xml_text = f.read()
+        return (Path(__file__).parent / "data" / datafile).read_text("utf-8")
 
     monkeypatch.setattr(QtArchives, "_download_update_xml", _mock)