Remove Django 1.6 through 1.10 compatibility.
Review Request #11627 — Created May 26, 2021 and submitted
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as it's no longer applicable in modern versions of Django.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.