Only allow decimal IPv4 addresses in IPv6 addresses.

According to http://www.ietf.org/rfc/rfc3986.txt the IPv4 address
at the end of IPv6 addresses may not have leading zeroes. This
enables IPv4Address to only accept decimal numbers, if that flag
is set.
In 2.x this is equivalent, but in 3.x: Why a list instead of a generator expression?