Our settings upgrade code is pretty old, and a bit crufty. While most of

it is unlikely to ever need to be run again, it's worth keeping it here

and maintainable than not. Particularly as we iterate on it.
Part of this change fixes improves how we output upgraded settings for

databases and cache backends.
The other part upgrades the postgresql_psycopg2 backend to

postgresql. A little history here: Originally, ages ago, Django had

two backends, and recommended moving postgresql to

postgresql_psycopg2, and we had upgrade code handling this. Then they

flipped this, and in Django 1.9 the psycopg2 variant became an alias,

which was then formally deprecated in 2.0, and removed in 3.0.
Review Board 4.0 doesn't particularly care which is used, but in order

to future-proof, we're handling the upgrades now, moving to the plain

postgresql backend.
Unit tests were added to check the settings upgrade machinery.

Unit tests pass on all supported versions of Python.