chore(deps): update dependency @types/sinon to v9 (#399)

This PR contains the following updates:

| Package | Type | Update | Change |
|---|---|---|---|
| [@types/sinon](https://togithub.com/DefinitelyTyped/DefinitelyTyped) | devDependencies | major | [`^7.5.2` -> `^9.0.0`](https://renovatebot.com/diffs/npm/@types%2fsinon/7.5.2/9.0.0) |

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packages/google-cloud-monitoring/package.json

@@ -47,7 +47,7 @@
   "devDependencies": {
     "@types/mocha": "^7.0.0",
     "@types/node": "^12.12.27",
-    "@types/sinon": "^7.5.2",
+    "@types/sinon": "^9.0.0",
     "c8": "^7.1.0",
     "codecov": "^3.6.5",
     "eslint": "^6.0.0",