Split function for host:port in standard lib

[Hartmut Goebel]

Michael Ströder schrieb:

> Examples IPv6 addresses:
> '::1:389' -> ('::1',389)
> '::1' -> ('::1',None)

These are wrong, see http://tools.ietf.org/html/rfc2732 ("Format for 
Literal IPv6 Addresses in URL's§).

Correct formats are:
[::1]:389
[::1]