Add IPv6 support to runserver_plus

The current version does not support commands like

    runserver_plus [::]:8080 
    runserver_plus example.org:8080

This is a backport from django/django@d7fa33af and django/django@6a32e253.

(Our use case: We run development servers in our private cloud, where they have only ipv6 global address. Testing changes with devices outside the local network is the simplest via ipv6.)