[3.15] gh-50966: Fix unbounded recursion in turtle drag handlers (GH-152626) (GH...

[3.15] gh-50966: Fix unbounded recursion in turtle drag handlers (GH-152626) (GH-152657)

TurtleScreenBase._update() redraws with cv.update(), which also reprocesses
input events, so a handler that moves the turtle (such as
screen.ondrag(turtle.goto)) reenters _update() for every queued event until
the interpreter crashes.  A reentrant _update() now only flushes drawing with
update_idletasks().
(cherry picked from commit 6f103fab178c07cbb5f701b8ad97e275b6eb6c4c)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>