Fix permission checkboxes for Django 3.1

wagtail/users/templatetags/wagtailusers_tags.py

@@ -1,5 +1,6 @@
 import itertools
 
+from django import VERSION as DJANGO_VERSION
 from django import template
 
 from wagtail.core import hooks
@@ -29,8 +30,7 @@ def format_permissions(permission_bound_field):
         ]
 
         (where 'checkbox' is an object with a tag() method that renders the checkbox as HTML;
-        this is an instance of django.forms.widgets.CheckboxChoiceInput on Django <1.11,
-        and a BoundWidget on Django >=1.11)
+        this is a BoundWidget on Django >=1.11)
 
         - and returns a table template formatted with this list.
 
@@ -41,10 +41,18 @@ def format_permissions(permission_bound_field):
 
     # iterate over permission_bound_field to build a lookup of individual renderable
     # checkbox objects
-    checkboxes_by_id = {
-        int(checkbox.data['value']): checkbox
-        for checkbox in permission_bound_field
-    }
+    if DJANGO_VERSION < (3, 1):
+        # checkbox.data['value'] gives the ID
+        checkboxes_by_id = {
+            int(checkbox.data['value']): checkbox
+            for checkbox in permission_bound_field
+        }
+    else:
+        # checkbox.data['value'] gives a ModelChoiceIteratorValue
+        checkboxes_by_id = {
+            int(checkbox.data['value'].value): checkbox
+            for checkbox in permission_bound_field
+        }
 
     object_perms = []
     other_perms = []