fix: add __future__.annotations to model declarations

Author: VladX09

python_client_generator/templates/models.py.mustache

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from datetime import datetime
 from enum import Enum
 from typing import Any, Dict, List, Literal, Optional, Union

tests/expected/fastapi_app_client/models.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from datetime import datetime
 from enum import Enum
 from typing import Any, Dict, List, Literal, Optional, Union

tests/expected/swagger_petstore_client/models.py

@@ -1,3 +1,5 @@
+from __future__ import annotations
+
 from datetime import datetime
 from enum import Enum
 from typing import Any, Dict, List, Literal, Optional, Union