configure: look for python3
configure: look for python3

Also don't check for ipaddr, it's been superseded by Python's built-in
ipaddress module.

Signed-off-by: Apollon Oikonomopoulos <ed81ed0a45ccd5c8869bda52e38353a6299ee39e@dmesg.gr>

ganeti/ganeti,mbakke/ganeti,mbakke/ganeti,ganeti/ganeti,mbakke/ganeti,ganeti/ganeti