[BUG] Bare connections cannot be used as context managers

Nessuno ha ancora preso questa 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).

Inizia dal punto di ingresso asyncpg.connect e confronta la connessione restituita con il comportamento di pool e del context manager pool.acquire mostrato nell’issue. Verifica il ciclo di vita previsto di async with per una connessione singola e aggiungi una copertura mirata per l’esempio segnalato; il lavoro è completato quando l’esempio entra ed esce senza AttributeError.

Scritto dal modello di indicizzazione a partire dal testo della issue.