Update dependency np to v6

Author: renovate-bot

package.json

@@ -38,7 +38,7 @@
     "esdoc-inject-script-plugin": "1.0.0",
     "esdoc-inject-style-plugin": "1.0.0",
     "esdoc-standard-plugin": "1.0.0",
-    "np": "2.20.1",
+    "np": "6.2.1",
     "nyc": "15.0.1"
   },
   "homepage": "https://aureooms.github.io/js-mincut",