Fix Bug #114293:

Fix Bug #114293:
    Strings are unpickled by calling eval on the string's repr. This
    change makes pickle work like cPickle; it checks if the pickled
    string is safe to eval and raises ValueError if it is not.

test suite modifications:
    Verify that pickle catches a variety of insecure string pickles
    Make test_pickle and test_cpickle use exactly the same test suite
    Add test for pickling recursive object