fix: Update f-string to pass tox py35

Author: Artur-at-work

checkbox-ng/plainbox/vendor/test_image_info.py

@@ -179,7 +179,7 @@ def test_invalid_dcd_formats(self):
 
         for example in invalid_examples:
             with self.assertRaises(
-                ValueError, msg=f"Should fail for: {example}"
+                ValueError, msg="Should fail for: {}".format(example)
             ):
                 image_info.dcd_string_to_info_iot(example)