We're now supporting Python 3.6 as a minimum for the 3.x range. This is

enforced in setup.py and manage.py, with the minimum versions now

defined in dependencies.py.
setup.py now permits packaging if setting the environment variable

RB_PY3_HAS_NO_OFFICIAL_SUPPORT=agreed. This helps with the

development and testing of the packaging while at the same time making

it clear to anyone doing so that we are not supporting this version yet.
It also bumps the minimum Django 1.11 release to the latest 1.11.20.

Tested running develop and bdist_wheel on Python 2.7, 3.6, and 3.7.
Tested that older Python versions triggered appropriate failures to run

setup.py and manage.py.