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/discord/models.py

@@ -123,7 +123,7 @@ class DiscordUser(models.Model):
                 )
                 if not guild_roles.has_roles(member_info['roles']):
                     raise RuntimeError(
-                        'Member %s has unknown roles: %s' % (
+                        'Member {} has unknown roles: {}'.format(
                             self.user,
                             set(member_info['roles']).difference(guild_roles.ids())
                         )