[py3] Replaced basestring by six.string_types.

django/forms/widgets.py

@@ -16,6 +16,7 @@ from django.utils.html import conditional_escape, format_html, format_html_join
 from django.utils.translation import ugettext, ugettext_lazy
 from django.utils.encoding import StrAndUnicode, force_unicode
 from django.utils.safestring import mark_safe
+from django.utils import six
 from django.utils import datetime_safe, formats
 from django.utils import six
 
@@ -522,7 +523,7 @@ class CheckboxInput(Widget):
         value = data.get(name)
         # Translate true and false strings to boolean values.
         values =  {'true': True, 'false': False}
-        if isinstance(value, basestring):
+        if isinstance(value, six.string_types):
             value = values.get(value.lower(), value)
         return value