Upgrade Django to 2.2.16 (9/1)

[hectcastro]

Django versions 3.1.1, 3.0.10, and 2.2.16 will be released on Tuesday, September 1, 2020 around 1000 UTC. They will fix two security defect with severity “low”.

See:

• https://docs.djangoproject.com/en/dev/internals/security/#how-django-discloses-security-issues
• https://github.com/azavea/docker-django/actions/runs/223853299 (slim builds are failing)