title: Exception creating a card in stripe
body: <p>I'm trying to add a card to a customer with the following code:</p>
  
  <pre><code>cu = stripe.Customer.retrieve(customer_id)
  cu.cards.create(card=token)
  </code></pre>
  
  <p>However, I am getting the following exception:</p>
  
  <pre><code>'ListObject' object has no attribute 'create'
  </code></pre>
  
  <p>It seems cu.cards does not have a "create" method.</p>
  
  <p>I'm using the 1.12.0 library for python.</p>
  
  <p>What am I doing wrong here?</p>