Declare Python 2 not to be supported

Mayavi has stopped testing against Python 2.7 in https://github.com/enthought/mayavi/pull/912. Since then changes on Mayavi may not work on Python 2 (e.g. changes in #983).

With that, there should be a notice as in which version of Mayavi drops support of Python 2, and possibly with an update of `setup.py` with `python_requires='>=3'` such that package installers can avoid the releases that don't support Python 2.