gh-116946: Implement the GC protocol for _tkinter tkapp and tktimertoken (GH-152310)

gh-116946: Implement the GC protocol for _tkinter tkapp and tktimertoken (GH-152310)

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

A pending timer is kept alive by the Tcl event loop, which fires it even
after the Python token is dropped, so it is treated as a GC root (only its
callback is traversed) and collecting it never cancels a live timer.  The
GC slots use a plain cast rather than the type-checking macro, since the
collector may visit a surviving object at shutdown after module clearing
has reset the global type pointers.  Deallocation cancels any pending timer
so its callback cannot run on freed memory.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Co-authored-by: Bénédikt Tran <10796600+picnixz@users.noreply.github.com>