DateTime with time zones

As a developer, I would like to have the ability to store the time zone associated to my `DateTime` objects.

Currently, they look like this in the Alembic migrations:

```python
sa.Column("created_at", sa.DateTime(), nullable=True),
```

But I would like to see:
```python
sa.Column("created_at", sa.DateTime(timezone=True), nullable=True),
```
