[Python-Dev] py34 makes it harder to read all of a pty

[Buck Golemon]

This code works as expected in python <3.4 (printing 'ok' then an error),
but only gives an error in python3.4.

```
import os
read_fd, write_fd = os.openpty()
os.write(write_fd, b'ok\n')
os.close(write_fd)

read_file = os.fdopen(read_fd, 'rb', 0)
print("read: %r" % read_file.read())
print("read: %r" % read_file.read())
```

This is due to the fix for issue21090 <http://bugs.python.org/issue21090>,
which aimed to un-silence errors which previously went unheard. The fix is
for me, as a user, to write a loop that uses os.read and interpretes EIO as
EOF. This is what I had hoped file.read() would do for me, however, and
what it used to do in previous pythons.
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