tp_free is NULL or PyObject_Del at the end.  Because it's a base type
it must call tp_free in its dealloc function, and because it's gc'able
it must not call PyObject_Del.

inherit_slots():  Don't inherit tp_free unless the type and its base
agree about whether they're gc'able.  If the type is gc'able and the
base is not, and the base uses the default PyObject_Del for its
tp_free, give the type PyObject_GC_Del for its tp_free (the appropriate
default for a gc'able type).

cPickle.c:  The Pickler and Unpickler types claim to be base classes
and gc'able, but their dealloc functions didn't call tp_free.
Repaired that.  Also call PyType_Ready() on these typeobjects, so
that the correct (PyObject_GC_Del) default memory-freeing function
gets plugged into these types' tp_free slots.