fix lint errors

judge/migrations/0098_default_changes.py

@@ -1,8 +1,8 @@
 # Generated by Django 2.2.9 on 2020-01-23 21:36
 
 import django.core.validators
-from django.db import migrations, models
 import django.db.models.deletion
+from django.db import migrations, models
 
 
 class Migration(migrations.Migration):

judge/migrations/0100_custom_checker.py

@@ -1,6 +1,6 @@
 # Generated by Django 2.2.9 on 2020-01-23 22:28
-
 from django.db import migrations, models
+
 import judge.models.problem_data
 import judge.utils.problem_data