Support JupyterLab 2.0

Cf jupyter-widgets/ipywidgets#2798 (comment)

Author: Zerline

js/package.json

@@ -51,7 +51,7 @@
   },
   "dependencies": {
     "@phosphor/application": "^1.6.0",
-    "@jupyter-widgets/base": "^2.0.1",
+    "@jupyter-widgets/base": "^2.0.1 || ^3",
     "@jupyter-widgets/controls": "^1.5.2",
     "@types/node": "^12.6.9",
     "ajv": "^6.10.2",