Fix unrecognized arguments error (#1327)

Author: asottile

src/tox/config/__init__.py

@@ -125,7 +125,7 @@ def add_testenv_attribute_obj(self, obj):
     def parse_cli(self, args, strict=False):
         args, argv = self.argparser.parse_known_args(args)
         if argv and (strict or WITHIN_PROVISION):
-            self.argparser.error("unrecognized arguments: %s".format(" ".join(argv)))
+            self.argparser.error("unrecognized arguments: {}".format(" ".join(argv)))
         return args
 
     def _format_help(self):