update code to reflect the new minimum python version 3.7

- update string format method
- remove redundant default arguments from function  calls
- remove unused imports
- remove unicode identifier from strings, it's default in py3 (see: https://stackoverflow.com/a/4182635/12201331)

allianceauth/authentication/migrations/0001_initial.py

@@ -1,5 +1,4 @@
 # Generated by Django 1.10.1 on 2016-09-05 21:38
-from __future__ import unicode_literals
 
 from django.conf import settings
 from django.db import migrations, models