Limit addr to (host, port) even for IPv6

The address of an IPv6 socket as returned by getsockname() is
a four-element tuple, rather than a two-element tuple as expected by
the existing pytest_localserver.smtp module code. In this commit I'm
extracting the first two elements of the tuple so that a Server's addr
attribute will be a 2-tuple of (host, port) regardless of whether
the underlying socket is an IPv4 or IPv6 socket.

Author: diazona

pytest_localserver/smtp.py

@@ -84,7 +84,7 @@ def _set_server_socket_attributes(self):
             assert hasattr(self, 'port')
             return
 
-        self.addr = self.server.sockets[0].getsockname()
+        self.addr = self.server.sockets[0].getsockname()[:2]
 
         # Work around a bug/missing feature in aiosmtpd (https://github.com/aio-libs/aiosmtpd/issues/276)
         if self.port == 0: