@takluyver's pexpect-u is unicode-ready, and we handle it, but
spawnb is not in `__all__` (at least in the 2.5 release), so our
check for it [here](https://github.com/ipython/ipython/blob/master/IPython/utils/_process_posix.py#L151)
fails, and we use the unicode output, calling `decode` on it directly,
which would raise UnicodeError on things like `ls` in a dir with unicode files.
This manually assigns `pexpect.spawnb`, not trusting that it will be included in
`from pexpect import *`.