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

[Roundup Robot]

Roundup Robot added the comment:

New changeset ec69518aeebc by Martin Panter in branch '2.7':
Issue #20699: Document that “io” methods should accept memoryview
https://hg.python.org/cpython/rev/ec69518aeebc

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