Refs #23919 -- Removed six.<various>_types usage

Thanks Tim Graham and Simon Charette for the reviews.

django/forms/boundfield.py

@@ -3,7 +3,6 @@ import warnings
 
 from django.forms.utils import flatatt, pretty_name
 from django.forms.widgets import Textarea, TextInput
-from django.utils import six
 from django.utils.deprecation import RemovedInDjango21Warning
 from django.utils.encoding import force_text
 from django.utils.functional import cached_property
@@ -63,7 +62,7 @@ class BoundField(object):
     def __getitem__(self, idx):
         # Prevent unnecessary reevaluation when accessing BoundField's attrs
         # from templates.
-        if not isinstance(idx, six.integer_types + (slice,)):
+        if not isinstance(idx, (int, slice)):
             raise TypeError
         return self.subwidgets[idx]