Upgrade to Django 3.0

- upgrades
  - Django 3.0.14
  - django-jsonfield 1.4.1 (from 1.2.0)
  - django-oauth-toolkit 1.4.1 (from 1.1.3)
    - Stopping here because later versions have changes to the
      underlying model to support OpenID Connect.  Presumably this can
      be dealt with via a migration in our project.
  - django-guid 2.2.1 (from 2.2.0)
  - django-debug-toolbar 3.2.4 (from 1.11.1)
  - python3-saml 1.13.0 (from 1.9.0)
  - xmlsec 1.3.12 (from 1.3.3)

- Remove our project's use of django.utils.six in favor of directly
  using six, in awx.sso.fields.

- Temporarily monkey patch six back in as django.utils.six, since
  django-jsonfield makes use of that import, and is no longer being
  updated.  Hopefully we can do away with this dependency with the new
  generalized JSONField brought in with Django 3.1.

- Force a json decoder to be used with all instances of JSONField
  brought in by django-jsonfield.  This deals with the 'cast to text'
  problem noted previously in our UPGRADE_BLOCKERS.

- Remove the validate_uris validator from the OAuth2Application in
  migration 0025, per the UPGRADE_BLOCKERS, and remove that note.

- Update the TEMPLATES setting to satisfy Django Debug Toolbar.  It
  requires at least one entry that has APP_DIRS=True, and as near as I
  can tell our custom OPTIONS.loaders setting was effectively doing
  the same thing as Django's own machinery if this setting is set.

requirements/updater.sh

@@ -18,7 +18,8 @@ generate_requirements() {
   # shellcheck disable=SC1090
   source ${venv}/bin/activate
 
-  ${venv}/bin/python3 -m pip install -U pip pip-tools
+  # FIXME: https://github.com/jazzband/pip-tools/issues/1558
+  ${venv}/bin/python3 -m pip install -U 'pip<22.0' pip-tools
 
   ${pip_compile} "${requirements_in}" "${requirements_git}" --output-file requirements.txt
   # consider the git requirements for purposes of resolving deps