Update Tcl/Tk on Windows to 8.6.9

Merged Ondrej Baranovič requested to merge branch/win-tcl8.6 into branch/default

Update _tkinter on Windows to use Tcl/Tk 8.6. I already updated the externals repository to Tcl/Tk 8.6.9 in externals@ceeb8272688e, externals@129f2c614205.

I went with 8.6.9 instead of the newer 8.6.10 or 8.6.11 to match CPython 3.7-3.9 and also because the CPython 3.10 PR updating to Tcl/Tk 8.6.10 had extra changes in lib-python, see https://github.com/python/cpython/pull/22405.

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