fix: Fixed _pickle.UnpicklingError (#576)

The error indicates an issue with unpickling data during the migration process. Specifically, it raises an `_pickle.UnpicklingError` with the message "invalid load key, '{'." The data being loaded may be corrupted or not in the expected format.

constance/migrations/0003_drop_pickle.py

@@ -40,7 +40,10 @@ def migrate_pickled_data(apps, schema_editor) -> None:  # pragma: no cover
             prefixed_key = f'{_prefix}{key}'
             value = _rd.get(prefixed_key)
             if value is not None:
-                redis_migrated_data[prefixed_key] = dumps(pickle.loads(value))  # noqa: S301
+                try:
+                    redis_migrated_data[prefixed_key] = dumps(pickle.loads(value))  # noqa: S301
+                except pickle.UnpicklingError:
+                    continue
         for prefixed_key, value in redis_migrated_data.items():
             _rd.set(prefixed_key, value)