'Gets username based on response data from a provider.
Each provider has different logic for username generation. Sadly,
this is not extracted into its own method in python-social-auth, so we
must provide a getter ourselves.
Note that this is the *initial* value the framework will attempt to use.
If it collides, the pipeline will generate a new username. We extract
it here so we can force collisions in a polymorphic way.'

def get_username(self):

    raise NotImplementedError
