Issue #20699: Merge io bytes-like fixes from 3.5

diff --cc Lib/_pyio.py
Simple merge

diff --cc Lib/test/test_io.py
Simple merge

diff --cc Modules/_io/bufferedio.c
Simple merge

diff --cc Modules/_io/bytesio.c
Simple merge

diff --cc Modules/_io/fileio.c
Simple merge

diff --cc Modules/_io/iobase.c
Simple merge