Database timezone ignored for timestamptz

还没有人认领这个 Issue。

* **asyncpg version**: 0.27.0
* **PostgreSQL version**: 15.1
* **Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
the issue with a local PostgreSQL install?**: local
* **Python version**: 3.10.9
* **Platform**: Linux
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: no
* **If you built asyncpg locally, which version of Cython did you use?**: 0.29.33
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: not tested, since unlikely

I would expect the `datetime.datetime` object has the same timezone as psql or python determines (`CET`).

Instead, asyncpg sets the timezone to `UTC`.

psql shell, displays UTC+1 = CET properly:
```
=> select now()::timestamptz;
2023-02-19 01:53:28.453411+01
```

python-shell:
```python
>>> loop = asyncio.new_event_loop()
>>> c = loop.run_until_complete(asyncpg.connect(database='lol'))
>>> loop.run_until_complete(c.fetch("select now()::timestamptz"))
[]
# ^ here, i'd expect it to be with tzinfo=datetime.timezone(datetime.timedelta(seconds=3600), 'CET')

# because python does the same thing when having timezoned timestamps:
>>> datetime.datetime.now().astimezone()
datetime.datetime(2023, 2, 19, 1, 57, 12, 260030, tzinfo=datetime.timezone(datetime.timedelta(seconds=3600), 'CET'))
```

首先，在 Linux 上使用 asyncpg 0.27.0 和 Python 3.10.9 重现报告的 PostgreSQL 15.1 timestamptz 行为，并将 psql 输出与返回的 datetime 进行比较。跟踪 timestamp 解码路径，并确定涵盖 timestamptz 值的现有测试。完成的标准是：返回的 datetime 保留预期的数据库本地时区行为，并且该回归问题已由测试覆盖。

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