Fix dbf dependency break for Python 2

Created by: rdmurphy

So the dbf package maintainers apparently don't care about versioning and introduced changes in dbf 0.95.010 that break Python 2 support. Because the requirements.txt defines the dependency as dbf>=0.95.004, csvkit tries to pick up the newer version.

dbf attempts to load the enum package:

File "dbf.py", line 67, in <module>

    from enum import Enum, IntEnum

ImportError: No module named enum

But that is currently only supported in Python 3.4. So the install just fails. A quick search didn't turn up anything that backports enum to Python 2. No bueno.