bug fix: added import errno to src/netns/subprocess_.py

src/netns/subprocess_.py

 #!/usr/bin/env python
 # vim:ts=4:sw=4:et:ai:sts=4
 
-import fcntl, grp, os, pickle, pwd, signal, select, sys, time, traceback
+import errno, fcntl, grp, os, pickle, pwd, signal, select, sys, time, traceback
 
 __all__ = [ 'PIPE', 'STDOUT', 'Popen', 'Subprocess', 'spawn', 'wait', 'poll',
         'get_user', 'system', 'backticks', 'backticks_raise' ]