Keybindings with the Shift modifier now work correctly. So do bindings

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.