Remove gtk imports from codebase

[WillAyd]

We have a few "import gtk" imports in the clipboards part of the code base but I cannot find any information as to what these actually are, so I assume these are especially outdated given our drop of Py27 support.

I think this used to be a part of PyGTK, but links to that (like http://www.pygtk.org/) seem to redirect now to PyGObject which has different import machinery anyway

[xcz011]

Hi @WillAyd, I just give a PR, and hope I understand you correctly. If not, please feel free to correct me. Thanks.