Sort-of backport to 2.1.3 (if we ever release it) of the following.

Sort-of backport to 2.1.3 (if we ever release it) of the following.
(The fix looks different, but does the same thing to the 2.1 GC code
that Neil's patch does to the 2.2 GC code.)

This is Neil's fix for SF bug 535905 (Evil Trashcan and GC interaction).

The fix makes it possible to call PyObject_GC_UnTrack() more than once
on the same object, and then move the PyObject_GC_UnTrack() call to
*before* the trashcan code is invoked.

BUGFIX CANDIDATE!