part 2 of Neil Schemenauer's GC patches:

part 2 of Neil Schemenauer's GC patches:

This patch modifies the type structures of objects that
participate in GC.  The object's tp_basicsize is increased when
GC is enabled.  GC information is prefixed to the object to
maintain binary compatibility.  GC objects also define the
tp_flag Py_TPFLAGS_GC.