which use the Space key.  Limit unmodified user keybindings to the
function keys.
Python Bug 775353, IDLEfork Bugs 755647, 761557

Improve error handling during startup if there's no Tkinter.

M NEWS.txt
M PyShell.py
M config-keys.def
M configHandler.py
M keybindingDialog.py

Backport candidate.