Create some ports that are assigned to Django 3.1 instead Django 2.2

[kai]

Summary

* Django 2.2 is currently the "default" version in the ports tree due its
  long term support until April 2022.  Thus ports that are assigned to that
  version will exist until then.

  The following ports are required to make the upgrade of net-mgmt/netbox to
  the 2.9 release possible because it requires Django 3.1 and provides no
  backwards compatibility for previous versions of Django.

  - www/py-dj31-django-rq
  - www/py-dj31-django-auth-ldap
  - www/py-dj31-django-cacheops
  - www/py-dj31-django-cors-headers
  - www/py-dj31-django-debug-toolbar
  - www/py-dj21-django-filter
  - www/py-dj31-django-js-asset
  - www/py-dj31-django-mptt
  - www/py-dj31-django-prometheus
  - www/py-dj31-django-redis
  - www/py-dj31-django-tables2
  - www/py-dj31-django-taggit
  - www/py-dj31-django-timezone-field
  - www/py-dj31-djangorestframework
  - www/py-dj31-drf-yasg

* Also limit the new ports to Python 3.6+ due the requirements of Django 3.1
  and add/update the related CONFLICTS_INSTALL entries.

Test Plan

• make index -> OK
• poudriere bulk -n -a -t -> OK
• poudriere bulk -> (114Ramd64 for py36,py37,py38,py39 flavors)

[sunpoet]

LGTM. Thanks!