[3.12] gh-75666: Tkinter: "unbind(sequence, funcid)" now only unbinds "funcid" (GH-111322) (GH-112802)

Previously, "widget.unbind(sequence, funcid)" destroyed the current binding
for "sequence", leaving "sequence" unbound, and deleted the "funcid"
command.

Now it removes only "funcid" from the binding for "sequence", keeping
other commands, and deletes the "funcid" command.
It leaves "sequence" unbound only if "funcid" was the last bound command.

(cherry picked from commit cc7e45cc572dd818412a649970fdee579417701f)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: GiovanniL <13402461+GiovaLomba@users.noreply.github.com>