How to register a user? develop, python 3

[armicron]

I've checked out a develop branch and tried to register a user.

from warrant import Cognito

u = Cognito("us-east-xxxxI", "zzzzzzzzz", client_secret="yyyyy")
u.add_base_attributes(email='test@gmail.com')
u.add_custom_attributes(state='virginia', city='Centreville')
u.register(username='xxx', password='qwerty234@$')

I've got a TypeError

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/arm/coding/python/cognito/warrant/warrant/__init__.py", line 303, in register
    attributes= dict(self.base_attributes.items() + self.custom_attributes.items())
TypeError: unsupported operand type(s) for +: 'dict_items' and 'dict_items'

Tested with two versions of python.
python -V
Python 3.4.5
python -V
Python 3.6.1

Seems that the code is tested only for python 2.7 and why not to update a Readme?