Line numbers were added to the IDLE editor two days ago and will appear in the upcoming 3.8.0a3 and later 3.7.5. For new windows, they are off by default, but this can be reversed on the Setting dialog, General tab, Editor section. For existing windows, there is a new Show (Hide) Line Numbers entry on the Options menu. There is currently no hotkey. One can select a line or bloc of lines by clicking on a line or clicking and dragging.
Some people may have missed Edit / Go to Line. The right-click context menu Goto File/Line works on grep (Find in Files) output as well as on trackbacks.