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/groupmanagement/models.py

@@ -113,7 +113,7 @@ class AuthGroup(models.Model):
 
     class Meta:
         permissions = (
-            ("request_groups", u"Can request non-public groups"),
+            ("request_groups", "Can request non-public groups"),
         )
         default_permissions = tuple()