Support IPv6

[fasterit]

Github has recently added two IPv6 networks to their hook servers. So they will at some point start using them.

Setting config supybot.plugins.Github.address :: or (::0) fails with a nice backtrace:

ERROR 2021-08-25T16:12:46 Failed to load Github:
Traceback (most recent call last):
  File "/limnoria/.local/lib/python3.9/site-packages/supybot/plugins/Owner/", line 190, in _loadPlugins
    plugin.loadPluginClass(irc, m)
  File "/limnoria/.local/lib/python3.9/site-packages/supybot/", line 153, in loadPluginClass
    cb = module.Class(irc)
  File "/limnoria/.local/lib/python3.9/site-packages/supybot/utils/", line 102, in __init__
    original__init__(self, *args, **kwargs)
  File "/limnoria/limnoria/plugins/Github/", line 85, in __init__
    self.httpd = server_class((self.address, self.port), RequestHandler.GithubHandler)
  File "/usr/lib/python3.9/", line 452, in __init__
    self.server_bind()
  File "/usr/lib/python3.9/http/", line 138, in server_bind
    socketserver.TCPServer.server_bind(self)
  File "/usr/lib/python3.9/", line 466, in server_bind
    self.socket.bind(self.server_address)
socket.gaierror: [Errno -2] Name does not resolve
ERROR 2021-08-25T16:12:46 Exception id: 0x75898

Would be nice to support "::" or "::0" to get a dual stack listening socket.
And probably there should be an address6 to bind to if address (address4?) is a qualified IPv4, similarly to what limnoria does for the supybot.servers.http.hosts4 and supybot.servers.http.hosts6 config values.