fix NameError

like in

,----
|    File "/home/ralf/local/lib/python2.7/site-packages/gevent/hub.py", line 271, in __init__
|     self.loop = loop_class(flags=loop, default=default)
|   File "/home/ralf/pypycore/pypycore.py", line 396, in __init__
|     if default or __SYSERR_CALLBACK is None:
`----