Remove type hinting from clean_model as workaround

Ref: fastapi/fastapi#10661

geojson_pydantic/base.py

@@ -53,7 +53,7 @@ def validate_bbox(cls, bbox: Optional[BBox]) -> Optional[BBox]:
         return bbox
 
     @model_serializer(when_used="always", mode="wrap")
-    def clean_model(self, serializer: Any, info: SerializationInfo) -> Dict[str, Any]:
+    def clean_model(self, serializer: Any, info: SerializationInfo):
         """Custom Model serializer to match the GeoJSON specification.
 
         Used to remove fields which are optional but cannot be null values.