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/corputils/migrations/0004_member_models.py

@@ -1,5 +1,4 @@
 # Generated by Django 1.10.5 on 2017-03-26 20:13
-from __future__ import unicode_literals
 
 from django.db import migrations, models
 import django.db.models.deletion
@@ -48,7 +47,7 @@ class Migration(migrations.Migration):
         ),
         migrations.AlterUniqueTogether(
             name='corpmember',
-            unique_together=set([('corpstats', 'character_id')]),
+            unique_together={('corpstats', 'character_id')},
         ),
         migrations.RunPython(convert_json_to_members, convert_members_to_json),
         migrations.RemoveField(