For subtle reasons, PyObject_GC_UnTrack() function must be called before
Py_TRASHCAN_BEGIN().  There have been a number of bugs over the years
related to not doing this particular dance just right.  Integrating
the PyObject_GC_UnTrack() call makes it harder to do things incorrectly.
That avoids some hard to find bugs (e.g. only triggered when object
nesting gets deep enough).

Extensions that still call PyObject_GC_UnTrack() explictly will still
work correctly but the call is unneeded after this change.  It would
still be needed for the extension to work correctly with older versions
of Python.

It seems slightly cleaner to have the BEGIN/END macros at the start and
end of the dealloc function body.