topicdiff.py 0.4: make script compatible with Python 3

[jspricke]

No description provided.

[flashcode]

Hi,

I think the script will not work with Python 2 any more because zip_longest is not available. For now all scripts must be compatible with Python 2 and 3, during the transition.

I suggest to import with a try/except, like that:

    try:
        from itertools import zip_longest
    except ImportError:
        from itertools import izip_longest as zip_longest

[jspricke]

updated, thanks for the proposal.