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/services/modules/teamspeak3/models.py

@@ -16,7 +16,7 @@ class Teamspeak3User(models.Model):
 
     class Meta:
         permissions = (
-            ("access_teamspeak3", u"Can access the Teamspeak3 service"),
+            ("access_teamspeak3", "Can access the Teamspeak3 service"),
         )