Use list rather than set (Python 2.6 compatibility)

Curly braces as a shorthand for set is a Python 2.7 feature. Changing to a list solves the issue.

I also suggest setting up a Tox build for Python 2.6.
Should not you use a set instead of a list to keep the semantics the same?
```
    if type(value) in set([datetime.datetime, datetime.date, datetime.time]):
```