[BUG] Bare connections cannot be used as context managers

Personne n'a encore pris cette issue.

* **asyncpg version**: 0.20.1
* **PostgreSQL version**: 10.3
* **Python version**: 3.8.2
* **Platform**: Windows
* **Do you use pgbouncer?**: No
* **Did you install asyncpg with pip?**: Yes

While under most use cases, people will want to use connection pools to manage their connections, there are times where using bare connections turns out to be more practical. However, while connection pools can be opened and closed via context managers:

```py
async with asyncpg.create_pool(user='postgres', command_timeout=60) as pool:
async with pool.acquire() as con:
await con.fetch('SELECT 1')
```

...it appears that bare connections created via `asyncpg.connect` do not support this:

```py
async with asyncpg.connect(user='postgres') as con:
await con.fetch('SELECT 1')
```

```
File "./test.py", line 120, in fetch_ip_list
async with asyncpg.connect(user="postgres") as con:
AttributeError: __aexit__
```

I personally like using `with` blocks to handle the scope of resources like this, so this is a step down for me in terms of ergonomics, not to mention rather surprising given that connections coming from connection pools do work as context managers (even though those are merely returned to the pool instead of closed).

Commencez par le point d’entrée asyncpg.connect et comparez la connexion renvoyée avec le comportement de pool et du gestionnaire de contexte pool.acquire présenté dans l’issue. Vérifiez le cycle de vie async with attendu pour une connexion seule et ajoutez une couverture ciblée pour l’exemple signalé ; c’est terminé lorsque l’exemple entre et sort sans AttributeError.

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