Retire Python 2

[pllim]

Python 2 will not be maintained past Jan 1, 2020 (see https://pythonclock.org/). Please remove all Python 2 compatibility and move this package to Python 3 only.

For conda recipe (including astroconda-contrib), please include the following to prevent packaging it for Python 2 (https://conda.io/docs/user-guide/tasks/build-packages/define-metadata.html?preprocessing-selectors#skipping-builds):

build: 
  skip: true  # [py2k] 

Please close this issue if it is irrelevant to your repository. This is an automated issue. If this is opened in error, please let pllim know!