Use UTC for `timestamp without time zone`

Nobody has claimed this yet.

`asyncpg` converts timestamps to `datetime.datetime`. If a column has type `timestamp without time zone` then the datetime object's `.tzinfo` is `None`. This means `datetime` implicitly treats this datetime as if it is local, and source code has to replace tzinfo for every column that has `timestamp without time zone`.

I'd expect that when developers specify no timezone it is intended to be UTC, otherwise timezone would be specified in the postgres datatype?

With this in mind it makes sense to provide UTC timezone type when creating `datetime.datetime` objects from columns that do not specify timezone (at least IMO).

Start by locating asyncpg's PostgreSQL timestamp conversion entry point and the tests covering timestamp without time zone values. Review the current naive-datetime behavior and its compatibility implications; done should be a documented, tested decision about whether these values receive UTC tzinfo.

Written by the indexing model from the issue text.