Add unicode datetime attribute

[NazarioJL]

This PR adds the UnicodeDatetimeAttribute attribute type for storing datetimes as 8601 ISO strings with offset.

Usage

from datetime import datetime

from pynamodb.models import Model
from pynamodb_attributes import UnicodeDatetimeAttribute


class MyModel(Model):
    key = UnicodeDatetimeAttribute(hash_key=True)


model = MyModel()
model.key = datetime(1980, 11, 20)
model.save()  # There is now a record with value "1980-11-20T00:00:00.000000-00:00"

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.

[NazarioJL]

@ikonst are we ok with running black on the whole project? If so, I'll go ahead and do that.

[ikonst]

Let's black it in a separate PR first? Totally supportive of black btw.

[NazarioJL]

@ikonst done! I will ship another PR where this gets blacked.

[NazarioJL]

@ikonst added 3.9 in the test matrix. I signed the CLA but it is not passing. Let me know if there's anything else I can do.

[NazarioJL]

@ikonst I will break this PR into 3 separate PRs

1. blacken the repo and fix the mypy tests
2. Address Python 3.9 deprecates _field_types from NamedTuple #32
3. Add the UnicodeDatetimeAttribute attribute

[NazarioJL]

💨

[NazarioJL]

@ikonst this should be ready for release 0.3.1

[NazarioJL]

🚢

[ikonst]

Thanks for bearing with me! :)