Perform some cleanup and state that configshell-fb is compatible with Python 3

It turns out that configshell-fb is ready for Python 3. Inspired by Johannes Dewender, I modified `setup.py` to run `2to3` when building for Python 3.

Moreover, targetcli is very close to being compatible with Python 3. There is one issue because the ethtool library does not exist on Python 3. But if you comment out `import ethtool`, you can run the `targetcli` command with Python 3.