str(User) infinite recursion on Python 3

[louisabraham]

The problem is there: https://github.com/charliewolf/pynder/blob/master/pynder/models/user.py#L95

    def __str__(self):
        return six.text_type(self).encode('utf-8')

six.text_type is str in Python 3 hence an infinite loop.
The best would be to use future.utils.python_2_unicode_compatible.