cPickle.c, load_build(): Taught cPickle how to pick apart

the optional proto 2 slot state.

pickle.py, load_build():  CAUTION:  Noted that cPickle's
load_build and pickle's load_build really don't do the same
things with the state, and didn't before this patch either.
cPickle never tries to do .update(), and has no backoff if
instance.__dict__ can't be retrieved.  There are no tests
that can tell the difference, and part of what cPickle's
load_build() did looked accidental to me, so I don't know
what the true intent is here.

pickletester.py, test_pickle.py:  Got rid of the hack for
exempting cPickle from running some of the proto 2 tests.

dictobject.c, PyDict_Next():  documented intended use.

Lib/pickle.py

@@ -1249,6 +1249,10 @@ class Unpickler:
                 # the instance variables.  This is a semantic
                 # difference when unpickling in restricted
                 # vs. unrestricted modes.
+                # Note, however, that cPickle has never tried to do the
+                # .update() business, and always uses
+                #     PyObject_SetItem(inst.__dict__, key, value) in a
+                # loop over state.items().
                 for k, v in state.items():
                     setattr(inst, k, v)
         if slotstate: