If one starts IDLE from a command-line console (python -m idlelib) or Python console (import idlelib.idle), sys.__stdout__ is the TextIOWraper for that console and .fileno() returns 1. .get_terminal_size() will then return the console size. The exception occurs when IDLE is started from an icon.
Implementing David's suggestion for shutil will be easy: insert just before the fileno call
if not sys.__stdout__: raise ValueError()
This is what os.get_terminal_size() raises on IDLE. Comments in the code in posixpath.c make it clear that this is intended guis and the low_level os function.
This came up today on Stackoverflow when someone tried to use matplotlib.pyplot, which calls os.get_terminal_size, on IDLE.
https://stackoverflow.com/questions/46921087/pyplot-with-idle
(Patching shutil will not help for the os call.)