Add compatibility with custom user models

Author: ProfessorLogout

ninja_apikey/models.py

@@ -29,4 +29,4 @@ def is_valid(self):
         return self.expires_at >= timezone.now()
 
     def __str__(self):
-        return f"{self.user.username}<{self.prefix}>"
+        return f"{self.user.get_username()}<{self.prefix}>"