Modify configure to explain how to bypass python version check if required.

As new versions of python come out it's possible that a user will be using a later one than our supported list. This PR adds the message in that situation telling people how they can bypass the check and to encourage raising an issue/PR to support the new version if it works. It also adds information about the lowest supported version with an indication that earlier ones are unlikely to work (I could just remove this check, but it's good to be nice to users!)

I don't think this will be too controversial unless there is an objection to the new lines not being wrapped at <80 characters :-)