Fixing python-modernize issues.

cms/lib/xblock/runtime.py

@@ -2,6 +2,9 @@
 XBlock runtime implementations for edX Studio
 """
 
+from __future__ import absolute_import
+
+import six
 from django.urls import reverse
 
 
@@ -14,7 +17,7 @@ def handler_url(block, handler_name, suffix='', query='', thirdparty=False):
         raise NotImplementedError("edX Studio doesn't support third-party xblock handler urls")
 
     url = reverse('component_handler', kwargs={
-        'usage_key_string': unicode(block.scope_ids.usage_id).encode('utf-8'),
+        'usage_key_string': six.text_type(block.scope_ids.usage_id).encode('utf-8'),
         'handler': handler_name,
         'suffix': suffix,
     }).rstrip('/')