bugs on sizeof(long)==8 machines.  pickle.py has no idea what it's
doing with very large ints, and variously gets things right by accident,
computes nonsense, or generates corrupt pickles.  cPickle fails on
cases 2**31 <= i < 2**32:  since it *thinks* those are 4-byte ints
(the "high 4 bytes" are all zeroes), it stores them in the (signed!) BININT
format, so they get unpickled as negative values.