The new version scheme was introduced in commit d2e4f63f03 ("T596: use a more
descriptive dev build version format.". It superseeded the old
999.something version numbers.
New version numbers for rolling releases are in the form of
vyos-1.2.0-rolling+201806062125-amd64.iso but the '+' sign will be
replaced by '%2B' when e.g. downloading the file via http(s), this may be
confusing.
Since we only support jessie as build host, and jessie knowingly does have
python3 (although not by default), we don't really need to worry about being
both 2 and 3 compatible.
