[notebook] Fix notebook unit test in py3
@@ -481,7 +481,7 @@ class TestNotebookApiMocked(object):
'snippet': json.dumps(json.loads(notebook_json)['snippets'][0]),
'format': 'csv'
})
- content = "".join(response)
+ content = b"".join(response)
assert_true(len(content) > 0)
