Changing test to work with django 2.1 and higher (#23573)

This is part of the django 2 upgrade

openedx/core/djangoapps/api_admin/tests/test_forms.py

 #pylint: disable=missing-docstring
 
-
+import django
 import ddt
 from django.test import TestCase
 
@@ -34,9 +34,13 @@ class ViewersWidgetTest(TestCase):
         """
         dummy_string_value = 'staff, verified'
         input_field_name = 'viewers'
-        expected_widget_html = u'<input type="text" name="{input_field_name}" value="{serialized_value}" />'.format(
+        extra_formating = ''
+        if django.VERSION < (2, 1):
+            extra_formating = ' /'
+        expected_widget_html = '<input type="text" name="{input_field_name}" value="{serialized_value}"{extra_formating}>'.format(
             input_field_name=input_field_name,
             serialized_value=dummy_string_value,
+            extra_formating=extra_formating,
         )
         output = self.widget.render(name=input_field_name, value=dummy_string_value)
         self.assertEqual(expected_widget_html, output)