text: Title: When mutation takes a list as an argument - passing in a list of the type does not work
  Body: I have an input type that takes a list as an argument.  I haven't seen an example of this use case so I just guessed that I could pass in a python list of the right type.  But I get the following error:
  
  `AttributeError: 'MyInput' object has no attribute 'items'`
  
  And, in fact, it is expecting a dict.  Here's an edited version of the code I use to run the mutation:
  
      list_arg = [
          gql.MyInput({"key1":"val1"})
          gql.MyInput({"key1":"val2"})
      ]
  
      op.my_mutation(my_list=list_arg)
  
  I'm assuming that passing a simple list into the argument is not the right way to go about it, but I'm not sure how to construct the list otherwise.
  
  Thoughts?