scripts/mailtest: fix socket family check.

The `socket` module has no `AddressFamily` attribute:
```
Traceback (most recent call last):
  File "/var/docker/scripts/mailtest", line 140, in <module>
    do_tls(smtp,smtp_sslv)
  File "/var/docker/scripts/mailtest", line 44, in do_tls
    if conn.sock.family == socket.AddressFamily.AF_INET:
AttributeError: 'module' object has no attribute 'AddressFamily'
```

However, there is `socket.AF_INET`.