Fix test_pickle, by reverting the string opcodes (S, T, U) to returning

Fix test_pickle, by reverting the string opcodes (S, T, U) to returning
strings, in Latin-1.  Bytes are once more pickled through bytes.__reduce__,
but now it returns "latin-1" as the second parameter.

Unfortunately this breaks datetime pickling.  I'll have to investigate
further; reverting Martin's changes doesn't seem to help.