Apply suggestion from @vstinner

Co-authored-by: Victor Stinner <vstinner@python.org>

Author: serhiy-storchaka

Lib/test/test_xml_etree.py

@@ -1072,8 +1072,8 @@ def test_parse_text_source(self):
         # gh-99064: The encoding declared in the document does not apply
         # to a source which is already decoded.
         def check(encoding, body):
-            xml = ("<?xml version='1.0' encoding='%s'?><xml>%s</xml>" %
-                   (encoding, body))
+            xml = (f"<?xml version='1.0' encoding='{encoding}'?>" 
+                   f"<xml>{body}</xml>")
             with self.subTest(encoding=encoding):
                 self.assertEqual(ET.parse(io.StringIO(xml)).getroot().text,
                                  body)