Issue 2402: get rid of warnings in regrtest with -3

Running the test suite with -3 enabled issues a load of warnings. The
Python 2.6 library should not issue any py3k warnings.

No, *correct usage* of the stdlib should not result in Py3k warnings.
Old APIs should still be tested, which I suppose is part of the reason
that there are so many warnings from testing.

Fair enough. I agree that the deprecated APIs should still be tested.
But there are a lot of other warnings, e.g. where the standard lib uses
``has_key`` instead of ``in``. These should be removed. The only
warnings should be those that are actively testing deprecated APIs.

Testing of deprecated APIs should probably temporarily suppress the Py3K
warnings while they're executing, though that's not really crucial.

Reducing the flood of warnings is really worthwhile -- I just fixed a
bug in test_atexit which only appeared when -3 was supplied.

I made a little decorator which silences py3k warnings. It could be
useful for test.test_support.

def silence_py3k(func):
    def decorator(*args, **kwargs):
        warnings.simplefilter("ignore", warnings.DeprecationWarning)
        func(*args, **kwargs)
        warnings.simplefilter("default", warnings.DeprecationWarning)
    if sys.flags_py3kwarning:
        return decorator
    else:
        return func

Brett has been doing this.