I'm a bit puzzled that both str and bytes are accepted by os.write() in
py3k:

>>> os.write(1, "foo\n")
foo
4
>>> os.write(1, b"foo\n")
foo
4

posix_write() uses s* 

s* (string, Unicode, or any buffer compatible object) [Py_buffer *]
http://docs.python.org/dev/3.0/c-api/arg.html

IMHO os.write should not accept unicode and convert it to default
encoding. The low level os functions are all about bytes.

Agreed.  But we need to tread carefully -- fixing this might break other
stuff that has silently relied on it.  Better try it ASAP.

Here is a patch. I've covered all direct uses of os.write() in the
standard library, but perhaps there are indirect uses. Please review.

Assuming you've run the test suite, go ahead and apply.

Thanks, committed in r66469.