Python default version

[Ryan Schmidt]

On Mar 19, 2018, at 10:52, Ryan Schmidt wrote:

> I'm proposing that the portgroup should require ports to specify python.versions. Don't offer a default. Identify and update all ports that use the python portgroup but that don't set python.versions. Set python.versions to 36 in those ports and increase their revisions. I'm also proposing that the default subport would change from the 27 subport if present to the latest 3x subport present.

The default subport is controlled by python.default_version, and we should require ports to specify that too.