[docs] [issue33280] Update link to Tcl/Tk 8.6 man pages in tkinter.rst

[Serhiy Storchaka]

Serhiy Storchaka <storchaka+cpython at gmail.com> added the comment:

Different builds of Python can use different versions of Tcl/Tk (as well as other external libraries). Tkinter supports features added in versions newer than minimal.

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