[1.7.x] Fixed flake8 warnings.

Backport of 9a922dcad1 from master
[1.7.x] Updated timezone docs to use timezone.now()

Thanks James Cleveland for the report and Aymeric for suggesting
the solution.

Backport of da59902250 from master
[1.7.x] Used JsonResponse in CBV examples.

Thanks Hiroki Kiyohara and Tim Graham for the reviews.

Backport of 7579080899 from master