Fix IPv6 support for strange setups

A few boxes have a strange environment:
IPv6 enabled kernel and IPv6 enabled Python but only an ancient Twisted.
E.g. old OpenPLi on DM7025 still has Twisted 8.2.0, but Python and kernel are IPv6 enabled.
IPv6 support in Twisted started in 12.0.0 (Which is enough for what we need).