title: How to specify "nullable" return type with type hints
body: <p>Suppose I have a function:</p>
  
  <pre><code>def get_some_date(some_argument: int=None) -&gt; %datetime_or_None%:
      if some_argument is not None and some_argument == 1:
          return datetime.utcnow()
      else:
          return None
  </code></pre>
  
  <p>How do I specify the return type for something that can be <code>None</code>?</p>