turtledemo: add root arg to PanedWindow call

[terryjreedy]

import tkinter as tk
tk._support_default_root=False
from turtledemo.__main__ import main
main()

currently fails at line 165: pane = PanedWindow(orient=HORIZONTAL, ... with RuntimeError: No master specified and tkinter is configured to not support default root. Since all other widget calls include a master argument, adding 'root' here fixes the failure.

Linked PRs

• gh-111092: Make turtledemo run without default root enabled #111093
• [3.12] gh-111092: Make turtledemo run without default root enabled (GH-111093) #111095
• [3.11] gh-111092: Make turtledemo run without default root enabled (GH-111093) #111096