* :mod:`pickle` is now more compatible with Python 2.x when using a

2.x-compatible protocol (that is, protocol 2 or lower), through translation

of some standard library module names to or from their Python 2.x

equivalents.

This means that more (protocol 2 or lower) pickles produced by Python 3.1

will be reusable by Python 2.x, and vice-versa. Standard set objects are

an example of this improvement.

This has the (unfortunate but unavoidable) side effect that some

protocol 2 pickles produced by Python 3.1 won't be readable with

Python 3.0. The latest pickle protocol, protocol 3, should be used when

migrating data between Python 3.x implementations, as it doesn't attempt

to remain compatible with Python 2.x.

(Contributed by Alexandre Vassalotti and Antoine Pitrou, :issue:`6137`.)