TCPConnector ttl_dns_cache type hint

Hello,

```
class TCPConnector
def __init__(
....
ttl_dns_cache: int=10
...)
```

but documentation (and code so):

> ttl_dns_cache (int) –
> 
> expire after some seconds the DNS entries, None means cached forever. By default 10 seconds.

Can you annotate as Optional[int], please?