improve IPv6 support

[totaam]

Useful for quic #3376, which seems to want IPv6 sockets, at least on some platforms.

This ticket is just for recording changes already made:

• 551f135 + 3120eda python-zeroconf is now the default mdns backend but it still has issues with IPv6 addresses: https://github.com/python-zeroconf/python-zeroconf states that "Listening on localhost (::1) does not work. Help with understanding why is appreciated."
• 68aa5c1
• 07d38e2
• minor: d26c599