[BUG] Bare connections cannot be used as context managers

Ninguém assumiu esta issue ainda.

* **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).

Comece pelo ponto de entrada asyncpg.connect e compare a conexão retornada com o comportamento de pool e do gerenciador de contexto pool.acquire mostrado na issue. Verifique o ciclo de vida esperado de async with para uma conexão independente e adicione cobertura focada para o exemplo relatado; estará concluído quando o exemplo entrar e sair sem AttributeError.

Escrita pelo modelo de indexação a partir do texto da issue.