Fix some later Python3 and GTK3 deprecation issues

[swolegoal]

I updated the code to work with Python 3, as well as later versions of the Python GTK bindings. There are still a few deprecation warnings that still remain which will be fixed in future commits.

[swolegoal]

Please let me know if you have any questions, comments, or concerns.

Thanks!

[ebruck]

Thanks for doing this update!