Use UTC for `timestamp without time zone`

还没有人认领这个 Issue。

`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).

首先定位 asyncpg 的 PostgreSQL 时间戳转换入口点，以及涵盖 timestamp without time zone 值的测试。检查当前 naive datetime 行为及其兼容性影响；完成标准是形成一项有文档记录且经过测试的决定，明确这些值是否接收 UTC tzinfo。

由索引模型根据 Issue 内容生成。