In Python binding, get_children (and get and exists, and probably others) with expired session doesn't raise exception properly

In Python binding, get_children (and get and exists, and probably others) with expired session doesn't raise exception properly.

>>> zookeeper.state(h)
-112
>>> zookeeper.get_children(h, '/')
Traceback (most recent call last):
File "<console>", line 1, in <module>
SystemError: error return without exception set

Let me know if you'd like me to work on a patch.