Make unicodecsv importable under Python 3

I realize that `unicodecsv` is not necessary and not usable in Python 3. However, I'd like to be able to install and import `unicodecsv` in a project that supports Python 2 and 3. Currently, installation breaks under Python 3 because `unicodecsv` can't be imported.

The following minimal changes allow an installation under Python 3:
- exception catching compatible with Python 2.5 - 3
- catch ImportError for non-existing dependencies