Remove support for Python 2 after Jan 1st, 2020

[jvkersch]

Edit: adapted from https://github.com/enthought/traits/issues/519

Python 2 reaches its end-of-life on January 1st, 2020.

The next major release of Chaco that lands after Jan 1st 2020 will drop support for Python 2.7. To reflect the backwards compatibility breakage, this release will also get a major version number bump, probably to 5.0.0 (though if there's a reason for a 5.x release between now and January, the Python 3-only version could end up being 6.0.0 instead).

This issue does not address support for Python 3.5, whose EOL is scheduled for September 2020.