upgrade dependencies

Author: mbostock

package.json

@@ -42,7 +42,7 @@
   "devDependencies": {
     "d3-color": "1 - 3",
     "eslint": "7",
-    "mocha": "8",
+    "mocha": "9",
     "rollup": "2",
     "rollup-plugin-terser": "7"
   },