title: Object Is Not JSON Serializable When Using JSON Dumps
body: <p>I am using an async method to return an array of objects and I am getting a "TypeError:  is not JSON serializable" error and I am not sure how to correct it. Here is my code:</p>
  
  <pre><code>async def fetch(session, url):
      async with session.get(url) as response:
          return await response.text
  
  class NewStoriesHandler(tornado.web.RequestHandler):
      async def get(self):
          self.set_header("Access-Control-Allow-Origin", "*")
          response = requests.get(
          "https://hacker-news.firebaseio.com/v0/newstories.json?print=pretty")
          data = response.json()
          story_list = []
          async with aiohttp.ClientSession() as session:
             for url in data:
                 story_list.append(fetch(session, url))
  
          print(story_list)
          self.write(json.dumps(story_list, default=json_util.default))
          self.finish()
  </code></pre>
  
  <p>I tried to return Json back to the method but it was unsuccessful and I not sure what it is expecting back. The loop return an object for each call and appends it to the array.</p>