gh-116946: Implement the GC protocol for _tkinter tkapp and tktimertoken

[serhiy-storchaka]

The internal _tkinter.tkapp and _tkinter.tktimertoken types are heap types implemented in C but never implemented the garbage collector protocol, so reference cycles through a Tcl interpreter's trace function or a timer handler's callback could not be collected.

They now implement tp_traverse and tp_clear, set Py_TPFLAGS_HAVE_GC, allocate through the type's tp_alloc, and untrack on deallocation.

Two design points differ from the earlier, reverted attempt (gh-138331, reverted by gh-138807 over the gh-138789/gh-138791 regression):

• A pending timer is kept alive by the Tcl event loop, which fires it even after the Python token is dropped. It is therefore treated as a GC root -- only its callback is traversed, not the extra reference owned by the event loop -- so collecting it never cancels a live timer. test_timer_fires_after_gc guards this.
• The GC slots use a plain cast rather than the type-checking macro, because the collector may visit a surviving object during interpreter shutdown, after module clearing has reset the global type pointers (the source of the gh-138791: fix crash when visiting a non-consumed timer handler #138789 shutdown crash). test_pending_timer_at_shutdown guards this.

This is independent of and complementary to gh-80937 / #152294: it does not traverse the command table, so it does not collect the createcommand cycle, and that fix does not collect interpreter/timer cycles.

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