[3.13] gh-101830: Fix Tcl_Obj to string conversion (GH-120884) (GH-120905)

Accessing the Tkinter object's string representation no longer converts
the underlying Tcl object to a string on Windows.
(cherry picked from commit f4ddaa396715855ffbd94590f89ab7d55feeec07)

Co-authored-by: Serhiy Storchaka <storchaka@gmail.com>