Remove Django 1.6 through 1.10 compatibility.
Review Request #11627 — Created May 26, 2021 and submitted — Latest diff uploaded
This drops all code specific to versions of Django prior to 1.11,
slimming things down and making it easier to move forward with newer
chainable_select_related_queryset()method has also been removed,
as it's no longer applicable in modern versions of Django.
Unit tests pass on Django 1.11.