A minor issue found via static analysis

[tzickel]

ExceptionPexpect is not defined.

ptyprocess/ptyprocess/_fork_pty.py

Line 66 in 8f1ba73

raise ExceptionPexpect("OSError of errno.ENXIO should be raised.")

Found using cython and
https://gist.github.com/tzickel/ea4541b12c1298a8092c3ec5043ef77e

pip install -I --root temp_ptyprocess ptyprocess
python checkcython.py temp_ptyprocess

[takluyver]

Thanks! #36 should fix it.