Impossible to select arbitrary column when using SQLAlchemy

Personne n'a encore pris cette issue.

* **asyncpg version**: asyncpg==0.21.0
* **PostgreSQL version**: 13
* **Python version**: 3.6
* **Platform**: Alpine Linux (3.8)
* **Do you use pgbouncer?**: no
* **Did you install asyncpg with pip?**: yes
* **If you built asyncpg locally, which version of Cython did you use?**: N/A
* **Can the issue be reproduced under both asyncio and
[uvloop](https://github.com/magicstack/uvloop)?**: I don't know, I'm using uvloop right now

I'm trying to use `asyncpg` with `SQLAlchemy` on a project of mine. However, the following constructs to select a subset of columns in the underlying table doesn't work:

```python
query = mytable.select(mytable.c.date)
results = await database.fetch_all(query)
```

This raises the following error:

```
File "./main.py", line 140, in post_telemetry
tel_results = await vmo.database.fetch_all(tel_query)
File "/usr/local/lib/python3.6/site-packages/databases/core.py", line 140, in fetch_all
return await connection.fetch_all(query, values)
File "/usr/local/lib/python3.6/site-packages/databases/core.py", line 239, in fetch_all
return await self._connection.fetch_all(built_query)
File "/usr/local/lib/python3.6/site-packages/databases/backends/postgres.py", line 160, in fetch_all
rows = await self._connection.fetch(query, *args)
File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 443, in fetch
return await self._execute(query, args, 0, timeout)
File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 1446, in _execute
query, args, limit, timeout, return_status=return_status)
File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 1454, in __execute
return await self._do_execute(query, executor, timeout)
File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 1466, in _do_execute
stmt = await self._get_statement(query, None)
File "/usr/local/lib/python3.6/site-packages/asyncpg/connection.py", line 351, in _get_statement
statement = await self._protocol.prepare(stmt_name, query, timeout)
File "asyncpg/protocol/protocol.pyx", line 163, in prepare
asyncpg.exceptions.DatatypeMismatchError: argument of AND must be type boolean, not type timestamp with time zone
```

As far as I can't tell, `asyncpg` doesn't seem to support this feature [that is present in SQLAlchemy since version 1.4](https://docs.sqlalchemy.org/en/14/core/tutorial.html#selecting-specific-columns), but I may be wrong.

Let me know if you need additional information.

Commencez par reproduire la requête de sélection de colonnes de SQLAlchemy avec les versions signalées de Python, PostgreSQL et des paquets, puis inspectez le SQL transmis à asyncpg via databases. Confirmez si la requête générée provoque l’erreur d’incompatibilité de types ; le travail est considéré comme terminé lorsque la sélection d’un sous-ensemble de colonnes s’exécute correctement.

Rédigé par le modèle d'indexation à partir du texte de l'issue.