Fix tkinter.ttk with Tcl/Tk 9.0

[serhiy-storchaka]

There are two issues in tkinter.ttk with Tcl/Tk 9.0.

• Using the Variable methods trace_variable() and trace_vdelete(). They wrap deprecated Tcl commands which were removed in Tcl 9.0. New methods added in Add new methods to trace Tkinter variables #66313 should be used instead (added in Python 3.6 and supported in Tcl 8.4 or older).
• Combobox.current() fails because the underlying Tk command returns now an empty string instead of -1 for empty combobox. It would be right to translate it to None, but -1 is more backward compatible. In future we can change this method to return None.

Linked PRs

• gh-120211: Fix tkinter.ttk with Tcl/Tk 9.0 #120213
• [3.13] gh-120211: Fix tkinter.ttk with Tcl/Tk 9.0 (GH-120213) #120215
• [3.12] gh-120211: Fix tkinter.ttk with Tcl/Tk 9.0 (GH-120213) #120216