NullBooleanField is deprecated since Django 3.1 (#71619)

It must be replaced by BooleanField(null=True).

src/authentic2/a2_rbac/fields.py

@@ -15,10 +15,10 @@
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
 from django import forms
-from django.db.models import NullBooleanField
+from django.db.models import BooleanField
 
 
-class UniqueBooleanField(NullBooleanField):
+class UniqueBooleanField(BooleanField):
     """BooleanField allowing only one True value in the table, and preventing
     problems with multiple False values by implicitely converting them to
     None."""
@@ -28,10 +28,10 @@ class UniqueBooleanField(NullBooleanField):
         kwargs['blank'] = True
         kwargs['null'] = True
         kwargs['default'] = False
-        super(NullBooleanField, self).__init__(*args, **kwargs)
+        super().__init__(*args, **kwargs)
 
     def deconstruct(self):
-        name, path, args, kwargs = super(NullBooleanField, self).deconstruct()
+        name, path, args, kwargs = super().deconstruct()
         del kwargs['null']
         del kwargs['blank']
         del kwargs['unique']
@@ -59,4 +59,4 @@ class UniqueBooleanField(NullBooleanField):
         else:
             defaults = {'form_class': forms.BooleanField}
         defaults.update(kwargs)
-        return super(NullBooleanField, self).formfield(**defaults)
+        return super().formfield(**defaults)

src/authentic2/a2_rbac/migrations/0025_auto_20210622_1132.py

@@ -13,9 +13,12 @@ class Migration(migrations.Migration):
         migrations.AlterField(
             model_name='organizationalunit',
             name='user_can_reset_password',
-            field=models.NullBooleanField(
+            field=models.BooleanField(
                 choices=[(None, 'System default'), (True, 'Yes'), (False, 'No')],
                 verbose_name='Users can reset password',
+                default=None,
+                blank=True,
+                null=True,
             ),
         ),
     ]

src/authentic2/a2_rbac/models.py

@@ -126,8 +126,12 @@ class OrganizationalUnit(AbstractBase):
 
     admin_perms = GenericRelation('Permission', content_type_field='target_ct', object_id_field='target_id')
 
-    user_can_reset_password = models.NullBooleanField(
-        verbose_name=_('Users can reset password'), choices=USER_CAN_RESET_PASSWD_CHOICES
+    user_can_reset_password = models.BooleanField(
+        verbose_name=_('Users can reset password'),
+        choices=USER_CAN_RESET_PASSWD_CHOICES,
+        null=True,
+        default=None,
+        blank=True,
     )
 
     user_add_password_policy = models.IntegerField(