Fix CallableChoiceIterator import on Django 5.0+

wagtail/blocks/field_block.py

@@ -4,7 +4,6 @@ from decimal import Decimal
 from django import forms
 from django.db.models import Model
 from django.db.models.fields import BLANK_CHOICE_DASH
-from django.forms.fields import CallableChoiceIterator
 from django.utils.dateparse import parse_date, parse_datetime, parse_time
 from django.utils.encoding import force_str
 from django.utils.functional import cached_property
@@ -24,6 +23,12 @@ from wagtail.telepath import Adapter, register
 
 from .base import Block
 
+try:
+    from django.utils.choices import CallableChoiceIterator
+except ImportError:
+    # DJANGO_VERSION < 5.0
+    from django.forms.fields import CallableChoiceIterator
+
 
 class FieldBlock(Block):
     """A block that wraps a Django form field"""