Add the recently added python3targetconfig class. Also, we no longer
have the python 2 classes, remove all references to those.
(From yocto-docs rev: c63d88656e2fc5361c512d4d9b426260c3e339f3)
Signed-off-by: Paul Eggleton <paul.eggleton@microsoft.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>