title: Python type hints for types not yet defined
body: <p>How do I provide type hints for an alternative constructor for a class? For example if I have a method like this converting something to my new class,</p>
  
  <pre><code>class MyClass:
      @classmethod
      def from_string(cl: Type[????], s: str) -&gt; ????:
          p, q = some_function(s)
          return cl(p, q)
  </code></pre>
  
  <p>what should I write for <code>????</code>? It should by nature just be <code>MyClass</code>, but that variable is not defined yet at the time when the method definition is executed.</p>