title: Mypy error on dict of dict: Value of type "object" is not indexable
body: <p>I have the following dictionary on python:</p>
  <pre class="lang-py prettyprint-override"><code>dictionary = {
      'key1': 1,
      'sub_dict': {'key2': 0},
  }
  </code></pre>
  <p>When I run mypy on the following line:</p>
  <pre class="lang-py prettyprint-override"><code>print(dictionary['sub_dict']['key2'])
  </code></pre>
  <p>it raises the error <code>Value of type &quot;object&quot; is not indexable</code></p>