Stop using compatibility backports.
Review Request #12033 — Created Feb. 4, 2022 and submitted
We have a number of things that we backported from newer versions of
Django into Djblets. Because we no longer care about running on Django
1.x, we can just use all the upstream versions of these features.
While I was in here I fixed up some other small errors I noticed, mostly
related to unused imports.
Ran unit tests.