Improved comparison of byte strings.

When comparing bytes, the assertion message used to show the byte numeric value when showing the differences::

        def test():
    >       assert b'spam' == b'eggs'
    E       AssertionError: assert b'spam' == b'eggs'
    E         At index 0 diff: 115 != 101
    E         Use -v to get the full diff

It now shows the actual ascii representation instead, which is often more useful::

        def test():
    >       assert b'spam' == b'eggs'
    E       AssertionError: assert b'spam' == b'eggs'
    E         At index 0 diff: b's' != b'e'
    E         Use -v to get the full diff