Drop Python 2 support
You might have heard that Python 2 will not be supported beyond January 1st, 2020. By that time, Dune should also drop the support IMO. However, with Dune's Python efforts being in a state of evolution right now, we could spare us compatibility hassle by dropping the support even earlier than that. Opinions? @andreas.dedner?
Within the core modules, the only affected code is the cmake code searching for the python interpreter/libs and the convenience stuff that we built around the 2/3 issue (like forcing a major version etc.).