Make Python IDLE's icon simpler

[hugovk (Hugo van Kemenade)]

I see this one on macOS:

Which can be found in two places in Git and is at least 17 years old:

• https://github.com/python/cpython/blob/main/Mac/Icons/IDLE.icns
• https://github.com/python/cpython/blob/main/Mac/IDLE/IDLE.app/Contents/Resources/IDLE.icns