Use extra parameter instead of formatted line in logger

radicale/log.py

@@ -55,5 +55,5 @@ def start():
         if debug:
             LOGGER.setLevel(logging.DEBUG)
             LOGGER.debug(
-                "Logging configuration file '%s' not found, using stdout." %
-                filename)
+                "Logging configuration file '%s' not found, using stdout.",
+                extra=filename)