[pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci

src/tox/config/loader/api.py

@@ -133,7 +133,8 @@ def load(  # noqa: PLR0913
             if isinstance(converted, list) and isinstance(appends, list):
                 converted += appends
             else:
-                raise ValueError("Only able to append to lists")
+                msg = "Only able to append to lists"
+                raise ValueError(msg)
         return converted
 
     def build(  # noqa: PLR0913