gh-116946: fully implement GC protocol for _tkinter.Tk{app,tt}Object

[picnixz]

• tkinter.Tcl_Obj is empty but not immutable so we make it immutable to avoid the needs of GC protocol.
• _tkinter.tktimertoken and _tkinter.tkapp are not immutable but they are not empty either as they have a strong reference to a callable, so we should still make them support the full GC protocol.

• Issue: Heap types implemented in C must have Py_TPFLAGS_HAVE_GC and implement tp_traverse #116946