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/notifications/models.py

@@ -59,7 +59,7 @@ class Notification(models.Model):
     objects = NotificationManager()
 
     def __str__(self) -> str:
-        return "%s: %s" % (self.user, self.title)
+        return f"{self.user}: {self.title}"
 
     def save(self, *args, **kwargs):
         # overriden save to ensure cache is invaidated on very call