bpo-38041: Refine IDLE Shell restart lines. (GH-15709)

bpo-38041: Refine IDLE Shell restart lines.  (GH-15709)

 Restart lines now always start with '=' and never end with ' ' and fill the width of the window unless that would require ending with ' ', which could be wrapped by itself and possible confusing the user.
(cherry picked from commit 38da805d563422cf1bb9cd9be24c73806840fe30)

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>