Fix datetime serialization format

New, bumped version of chemas includes
timezone.UTC in every property of
HiveDateTime type.

When calling .isoformat() on datetime
object which has `tzinfo` set
(from .isoformat() documentation):
> If self.tzinfo is not None, the UTC offset is also attached,
> giving a full format of 'YYYY-MM-DD HH:MM:SS.mmmmmm+HH:MM'.