Removal of python 2.7 support

[lingster]

Given the retirement of legacy python 2.7 at the end of 2019. I'd like to propose that we remove python 2.7 support in django-alexa and to focus on python 3.6 and upwards. Many other packages such as Django 2.x officially no longer support python 2.7.

This will make it easier to maintain this project, unless there's any objections, I'll migrate the project accordingly.