Fix: we don't know output type of deserializer

Be more explicit about the fact that we don't know the output of deserialized output

Author: vidartf

packages/base/src/widget.ts

@@ -655,9 +655,9 @@ export class WidgetModel extends Backbone.Model {
   static _deserialize_state(
     state: Dict<BufferJSON>,
     manager: IWidgetManager
-  ): Promise<utils.Dict<BufferJSON>> {
+  ): Promise<utils.Dict<unknown>> {
     const serializers = this.serializers;
-    let deserialized: Dict<PromiseLike<BufferJSON> | BufferJSON>;
+    let deserialized: Dict<unknown>;
     if (serializers) {
       deserialized = {};
       for (const k in state) {