fix auto values for inserted field

cntmanage/flightslot/models/weekpref.py

@@ -1,5 +1,5 @@
 from django.db import models
-from datetime import date, datetime
+from datetime import date
 
 from ..models.students import Student
 
@@ -26,7 +26,7 @@ class WeekPreference(models.Model):
 
     inserted = models.DateTimeField(
         null=False,
-        default=datetime.now(),
+        auto_now_add=True
     )
 
     def __str__(self):