Avoid using module state for Python 3, as the code is not ready for it.
Avoid using module state for Python 3, as the code is not ready for it.

The code before this change would fail badly when calling PyErr_SetString()
as SGIOError was NULL.

Since the code uses SGIOError as a global variable (and that's still
supported) describe the module as so. Also, define SGIOError properly in
the module so that it can be caught.

rosjat/python-scsi