Database timezone ignored for timestamptz

* **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'))
```

Start by reproducing the reported PostgreSQL 15.1 timestamptz behavior with asyncpg 0.27.0 and Python 3.10.9 on Linux, comparing psql output with the returned datetime. Trace the timestamp decoding path and identify the existing tests covering timestamptz values. Done means the returned datetime preserves the expected database-local timezone behavior and the regression is covered by a test.

Written by the indexing model from the issue text.