Add unicode datetime attribute and Python 3.9 compatibility

Add unicode datetime attribute (#33)

UnicodeDatetimeAttribute stores datetimes as 8601 ISO strings with offset.

The storage representation of this format will look something like:
{"key": {"S": "2020-11-22T03:22:33.444444-08:00"}}

The attribute by default will add an offset to UTC if not present and make it timezone aware. It also as options for normalizing the date to UTC (for caching purposes) and adds support for custom formatting.