Officially drop python2 support

[robinmatz]

Suggestion:

Python2 has been deprecated since January 2020. It is no longer actively maintained, and the majority of users have migrated to python3.
Moving on, some features, like type hints, will no longer support python2. Also compatibility with python2 will increase maintenance efforts for little outcome. Therefore I opt for dropping python2 support in the upcoming release.

What needs to be done

• Include a section in the README indicating supported python versions
• Alter the docker image and related documentation
• remove redundant # -- coding: utf-8 - declarations, since utf-8 is the default encoding in python3
• remove imports from future, since this package serves python2 > python3 migration purposes