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/0010_only_one_authservicesinfo.py

@@ -1,5 +1,4 @@
 # Generated by Django 1.10.1 on 2017-01-07 06:47
-from __future__ import unicode_literals
 
 from django.db import migrations
 
@@ -9,7 +8,7 @@ def count_completed_fields(model):
 def forward(apps, schema_editor):
     # this ensures only one model exists per user
     AuthServicesInfo = apps.get_model('authentication', 'AuthServicesInfo')
-    users = set([a.user for a in AuthServicesInfo.objects.all()])
+    users = {a.user for a in AuthServicesInfo.objects.all()}
     for u in users:
         auths = AuthServicesInfo.objects.filter(user=u)
         if auths.count() > 1: