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.

Objects/dictobject.c

@@ -642,7 +642,17 @@ PyDict_Clear(PyObject *op)
 		PyMem_DEL(table);
 }
 
-/* CAUTION:  In general, it isn't safe to use PyDict_Next in a loop that
+/*
+ * Iterate over a dict.  Use like so:
+ *
+ *     int i;
+ *     PyObject *key, *value;
+ *     i = 0;   # important!  i should not otherwise be changed by you
+ *     while (PyDict_Next(yourdict, &i, &key, &value) {
+ *              Refer to borrowed references in key and value.
+ *     }
+ *
+ * CAUTION:  In general, it isn't safe to use PyDict_Next in a loop that
  * mutates the dict.  One exception:  it is safe if the loop merely changes
  * the values associated with the keys (but doesn't insert new keys or
  * delete keys), via PyDict_SetItem().