After testing it on Linux (through the latest dev version, launched with `./.python -m idlelib`), the line numbers seem to be working quite well across several different fonts and font sizes. My only suggestion for minor visual improvement would be to increase the margin between the right side of the number and the line. 

While moving through a number of different fonts and sizes, I noticed a separate issue involving the window scaling based on the size of the font previews. This isn't an issue for smaller fonts, but when attempting to use larger fonts (upper 20s or into the 30s depending on the font in question) the 4 buttons ("Ok", "Apply", "Cancel", and "Help") in the settings menu (options > configure IDLE) are moved past the lower end of the screen, making them no longer visible to the user.

Either way, this change is pretty awesome. The only thing preventing me from using the IDLE more was the lack of line numbers, so I'll definitely be using it a lot more now. Thanks for the PR taleinat!