This appears to be a bug in IronPython - types.FunctionType resolves to a constructor call that throws a NotImplementedException
(see https://github.com/IronLanguages/main/blob/master/Languages/IronPython/IronPython/Runtime/PythonFunction.cs#L72).
It's old code, so I'm not sure why it does that. I've opened issue #35180 to track it.