handle OSError errno 97 when ipv6 is disabled

[PierluigiGreto]

Hi,
I was using your netconf server library and I came across this error:

Traceback (most recent call last):
  File "server_exmaple.py", line 4, in <module>
    ssh_server = server.SSHServer(server_ctl, host_key="tests/host_key")
  File "/root/pysshutil/sshutil/server.py", line 363, in __init__
    protosocket = socket.socket(proto, socket.SOCK_STREAM)
  File "/usr/lib64/python3.6/socket.py", line 144, in __init__
    _socket.socket.__init__(self, family, type, proto, fileno)
OSError: [Errno 97] Address family not supported by protocol
Deleting SSHServer(port=0)

This happens when ipv6 is disabled in the OS. (This pull request is related also to pull 7).
I managed to reproduce the error and this solution works fine for me. Please let me know what you think.
Thank you for your work.

[choppsv1]

LGTM

[choppsv1]

Thanks for the fix!