[issue20699] Document that binary IO classes work with bytes-likes objects

[Martin Panter]

Martin Panter added the comment:

I thought :class:`str` is better documented in Python 2, but I can write bytes if you prefer. I guess it is more consistent with the rest of io.rst.

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Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue20699>
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