Use % formatting instead of f-strings in __del__

F-strings can fail during cleanup. Use % formatting like
subprocess.Popen does for safer __del__ behavior.

Signed-off-by: Osama Abdelkader <[email protected]>

Author: osamakader

Lib/xml/etree/ElementTree.py

@@ -1271,7 +1271,7 @@ def close(self):
 
         def __del__(self, _warn=warnings.warn):
             if close_source:
-                _warn(f"unclosed file {source!r}",
+                _warn("unclosed file %r" % (source,),
                       ResourceWarning, source=self)
                 source.close()