This problem seems not to be correctly tested in Python. When Python code calls TreeNode() initializer as given by the problem, the actual initializer called is the TreeNode from the ordinary binary tree class but not the linked node version needed for this problem, so that an error is reported as "TreeNode has no attribute next".
For most people, I know that the solution can get around this without creating any new TreeNode. But for some personal programming patterns, I am used to create dummy nodes to firmly handle list/tree structures, which is the reason why I found this bug during the test phase.
In some way I hacked out how the Python test case was internally written. Using the initializer TreeLinkNode() instead of TreeNode() should solve this issue. I suggest that the Python commented code of this question get modified so that TreeLinkNode() is used, to avoid possible misleads.