Simplify parse_ip_network() when addr is a tuple
Simplify parse_ip_network() when addr is a string
Remove duplicate sanity checks in __setstate__

Sanity checks for version and prefixlen are already performed in
IPNetwork.__init__() and parse_ip_network(), no reason to duplicate them
here. In case of an error, the exception message is also much shorter (the
state of the entire IPSet could be some megabytes).