On some systems, localhost is only defined for 127.0.0.1 (e.g. Ubuntu
and Debian containers). However, there is code that hardcodes possible
values for localhost, making it possible to open an IPv6 socket for
localhost.
On those systems, the socket will be open but urllib3 will resolve
localhost *only* to 127.0.0.1, thus failing miserably to connect.
To resolve the situation, rather than defaulting to IPv6 we actually
resolve localhost and use the socket family of the first result. On my
current system (upcoming Ubuntu Plucky) if localhost=::1 is defined in
/etc/hosts it will come up as the first result, if not 127.0.0.1 will.
V2: Use self.port rather than a forgotten hardcoded port.
Fixes: f01628ca6b "fix localhost network tests on systems with IPv6"
