fix shell testutil

Author: lostella

src/gluonts/testutil/shell.py

@@ -21,7 +21,7 @@
 import typing
 import waitress
 from contextlib import closing, contextmanager
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 from multiprocessing.context import ForkContext
 from pathlib import Path
 from typing import Any, ContextManager, Dict, Iterable, List, Optional, Type
@@ -119,7 +119,7 @@ def free_port() -> int:
 class Server:
     env: ServeEnv
     forecaster_type: Optional[Type[Predictor]]
-    settings: Settings = Settings()
+    settings: Settings = field(default_factory=Settings)
 
     def run(self):
         flask_app = make_flask_app(