FYI, the size of the terminal associated with the C's stdout isn't related to the IDLE shell. For example, in Linux when I run IDLE from the GUI, the associated terminal size is 0x0.
On Windows, os.get_terminal_size uses the console API GetConsoleScreenBufferInfo. This can't work given IDLE has no attached console. Also, for a GUI app the Windows C runtime leaves the standard FILE streams uninitialized to an invalid file descriptor (-1), so Python's sys.__stdout__ is None. That's why you get an AttributeError complaining that NoneType (i.e. type(None)) has no attribute 'fileno'.
Currently shutil.get_terminal_size returns the fallback size when os.get_terminal_size(sys.__stdout__.fileno()) raises NameError or OSError. I think AttributeError and ValueError should be added to this list.