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

[Roundup Robot]

Roundup Robot added the comment:

New changeset bd41151a7429 by Martin Panter in branch '3.5':
Issue #20699: Document that “io” methods accept bytes-like objects
https://hg.python.org/cpython/rev/bd41151a7429

New changeset c1b40a29dc7a by Martin Panter in branch 'default':
Issue #20699: Merge io bytes-like fixes from 3.5
https://hg.python.org/cpython/rev/c1b40a29dc7a

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nosy: +python-dev

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