Update the dependencies and package install for Python 3.x.

Review Request #10530 — Created April 13, 2019 and submitted — Latest diff uploaded

Review Board

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.