title: Graphene mutation with list as input
body: <p>I have a graphene mutation like this:</p>
  
  <pre class="lang-py prettyprint-override"><code>
  class User(ObjectType):
      username = String()
  
  class ImportUsers(Mutation):
      class Arguments:
          users = List(User)
      Output = List(User)
  
      @staticmethod
      def mutation(root, info, users):
          ...
  </code></pre>
  
  <p>But graphene gives me the following error: <code>AssertionError: Mutations.importUsers(users:) argument type must be Input Type but got: [User].</code></p>
  
  <p>How can I have a mutation in graphene which accepts a list of objects?</p>