Update torndb.py

[xs23933]

Problem with Python 3.x AttributeError: 'module' object has no attribute 'izip'

[bdarnell]

2.x itertools.izip() is equivalent to 3.x zip(), not itertools.zip_longest(). The try clause here covers a lot of ground; it would be better to do something at top level like

try:
    from itertools import izip
except ImportError:
    izip = zip