JSON takes unicode as input.

In python2 there would be some auto coercion done so that the orginal
code here works but that's not the case in python 3.

common/test/acceptance/fixtures/base.py

@@ -166,10 +166,7 @@ class XBlockContainerFixture(StudioApiFixture):
         """
         Encode `post_dict` (a dictionary) as UTF-8 encoded JSON.
         """
-        return json.dumps({
-            k: v.encode('utf-8') if isinstance(v, six.string_types) else v
-            for k, v in post_dict.items()
-        })
+        return json.dumps(post_dict).encode('utf-8')
 
     def get_nested_xblocks(self, category=None):
         """