[Python-3000] Simplifying pickle for Py3k

[Neil Schemenauer]

I guess the library overhaul hasn't really started it but it would
be nice if the pickle module could get some work.  Today I'm trying
to efficiently store a class using pickle and the documentation is
making my head hurt.  I don't think the documentation itself is the
problem, just the fact that the rules are so complicated.

I guess there are several different solutions:

    * Remove backwards compatible stuff from the code and the
      documentation.  The downside is that old pickles could not be
      loaded.  Perhaps that's not a huge issue since the removal of
      old-style classes might already break old pickles.

    * Remove the backwards compatible stuff from the documentation
      only.  The would help people using the language but would
      still be a long term maintenance issue.

    * Leave the old code in but generate warnings when old pickle
      mechanisms are used.  Eventually the old stuff could be
      removed from the code.

    * Provide an "oldpickle" module the supports pre-3k pickles.

I think I like the warnings idea best.

  Neil