fix compatibility issue with Django-3.1

Author: jrief

djng/forms/angular_base.py

@@ -4,6 +4,7 @@
 import warnings
 
 from django.forms import forms
+from django.forms.boundfield import BoundField
 from django.http import QueryDict
 from django.utils.html import format_html, format_html_join, escape, conditional_escape
 from django.utils.encoding import force_text
@@ -148,7 +149,7 @@ def get_context(self, name, value, attrs):
  return context
 
 
-class NgBoundField(forms.BoundField):
+class NgBoundField(BoundField):
  @property
  def errors(self):
  """