Fix whitespaces.

biscuit/core/migrations/0001_initial.py

@@ -91,7 +91,7 @@ class Migration(migrations.Migration):
         migrations.AddField(
             model_name='group',
             name='parent_groups',
-            field=models.ManyToManyField(related_name='child_groups', to='core.Group', verbose_name='Parent groups'),
+            field=models.ManyToManyField(related_name='child_groups', to='core.Group', verbose_name='Parent groups', blank=True),
         ),
         migrations.AddField(
             model_name='group',

biscuit/core/models.py

@@ -150,7 +150,7 @@ class Group(SchoolRelated):
     owners = models.ManyToManyField('Person', related_name='owner_of')
 
     parent_groups = models.ManyToManyField('self', related_name='child_groups',
-        symmetrical=False, verbose_name=_('Parent groups'))
+        symmetrical=False, verbose_name=_('Parent groups'), blank=True)
 
     def __str__(self) -> str:
         return '%s (%s)' % (self.name, self.short_name)